The information ecology of social media and online communities∗

Tim Finin, Anupam Joshi, Pranam Kolari, Akshay Java, Anubhav Kale and Amit KarandikarUniversity of Maryland, Baltimore County, Baltimore MD 21250

Abstract

Social media systems such as weblogs, photo- and link-sharing sites, wikis and on-line forums are currentlythought to produce up to one third of new Web content.One thing that sets these “Web 2.0” sites apart from tra-ditional Web pages and resources is that they are inter-twined with other forms of networked data. Their stan-dard hyperlinks are enriched by social networks, com-ments, trackbacks, advertisements, tags, RDF data andmetadata. We describe recent work on building systemsthat use models of the Blogosphere to recognize spamblogs, find opinions on topics, identify communities ofinterest, derive trust relationships, and detect influentialbloggers.

IntroductionWeb-based social media systems such as blogs, wikis,media-sharing sites and message forums have becomean important new way to publish information, engagein discussions and form communities on the Internet.Their reach and impact is significant, with tens of mil-lions of people providing content on a regular basisaround the world. Recent estimates suggest that so-cial media systems are responsible for as much as onethird of new Web content. Corporations, traditional me-dia companies, governments and NGOs are working tounderstand how to adapt to them and use them effec-tively. Citizens, both young and old, are also discover-ing how social media technology can improve their livesand give them more voice in the world. We must betterunderstand the information ecology of these new publi-cation methods in order to make them and the informa-tion they provide more useful, trustworthy and reliable.

We are developing a model of information flow, in-fluence and trust on the Blogosphere and are exploringhow this model can be used to answer questions likethe following: how can blog communities be identified

∗Submitted to AI Magazine, special issue on net-works, Fall 2007. Corresponding author: Tim Finin,finin@umbc.edu, 410-455-3522, fax: 410-455-3969).Copyright c© 2007, American Association for Artificial Intel-ligence (www.aaai.org). All rights reserved.

Figure 1: Modeling influence and information flow on theBlogosphere and other social media systems requires attentionto many factors, including link structure, sentiment analysis,readership data, conversational structure, topic classification,and temporal analysis (figure from (Adamic & Glance 2005)).

based on a combination of topic, bias, and underlyingbeliefs; which authors and blogs are most influentialwithin a given community; from where do particular be-liefs or ideas originate and how do they spread; what arethe most trustworthy sources of information about a par-ticular topic; and what opinions and beliefs characterizea community and how do these opinions change.

The Blogosphere is part of the Web and thereforeshares most of it’s general characteristics. It differs,however, in ways that impact how it should be modeled,analyzed and exploited. The common model for thegeneral World Wide Web is as a directed graph of Webpages with undifferentiated links between pages. TheBlogosphere has a much richer network structure in thatthere are more types of nodes which have more kinds ofrelations between them. For example, the people whocontribute to blogs and author blog posts form a so-cial network with their peers, which can be induced bythe links between blogs. The blogs themselves form agraph, with direct links to other blogs through blogrollsand indirect links through their posts. Blog posts arelinked to their host blogs and typically to other blog

Draft preprint: Tim Finin, Anupam Joshi, Pranam Kolari, Akshay Java, Anubav Kale and Amit Krandikar,The Information ecology of social media and online communities, AI Magazine, 28(3), AAAI Press, 2007.

Blog Graph Influence Graph

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Figure 2: A graph of blogs and Web links between themcan be converted into an influence graph. A link fromu to v indicates that u is influenced by v. The edgesin the influence graph are reverse of the blog graph toindicate this influence. Multiple edges indicate strongerinfluence and are weighed higher

posts and Web resources as part of their content. A typ-ical blog post has a set of comments that link back topeople and blogs associated with them. Finally, the bl-ogosphere trackback protocol generates implicit linksbetween blog posts. Still more detail can be added bytaking into account post tags and categories, syndica-tion feeds, and semi-structured metadata in the form ofXML and RDF content.

In the rest of this article, we discuss our ongoing re-search in modeling the Blogosphere and extracting use-ful information from it. We begin by describing anoverarching task of discovering which blogs and blog-gers are most influential within a community or abouta topic. Pursuing this task uncovers a number of prob-lems that must be addressed, three of which we describein more detail. The first is recognizing spam in the formof spam blogs (splogs) and spam comments. The sec-ond is developing more effective techniques to recog-nize the social structure of blog communities. The finalone involves devising a better abstract model for the un-derlying blog network structure and how it evolves.

Modeling influence in the BlogosphereThe Blogosphere provides an interesting opportunity tostudy online social interactions including spread of in-formation, opinion formation and influence. Throughoriginal content and sometimes via commentary on top-ics of current interest, bloggers influence each other andtheir audience. We are working to study and charac-terize these social interaction by modeling the Blogo-sphere and providing novel algorithms for analyzing so-cial media content. Figure 2 shows a hypothetical bloggraph and its corresponding flow of information in theinfluence graph.

Studies on influence in social networks and collab-oration graphs have typically focused on the task ofidentifying key individuals who play an important rolein propagating information. This is similar to finding

authoritative pages on the Web. Epidemic-based mod-els like linear threshold and cascade models (Kempe,Kleinberg, & Tardos 2003; 2005; Leskovec et al. 2007)have been used to find a small set of individuals whoare most influential in social network. However, influ-ence on the Web is often a function of topic. For exam-ple, Engadget’s1 influence is in the domain of consumerelectronics and Daily Kos2 in politics. A post in the for-mer is unlikely to be very effective in influencing opin-ions on political issues even though Engadget is one ofthe most popular blogs on the Web.

The other related dimension of influence is reader-ship. With the large number of niches existing on theBlogosphere, a blog that is relatively low ranked can behighly influential in this small community of interest.In addition, influence can be subjective and based onthe interest of the users. Thus by analyzing the reader-ship of a blog we gain insights into the community thatis likely to be influenced by it.

We have implemented a system called Feeds ThatMatter3 (Java et al. 2007b) that aggregates subscriptioninformation across thousands of Bloglines users to au-tomatically categorize blogs into different topics. Blog-lines4 is a popular feed reader service that lets users tomanage subscriptions and monitor a number of feeds forany unread posts. Bloglines provides a feature allowsusers to share their subscriptions. We conduct a studyof the publicly listed OPML5 feeds from 83,204 usersconsisting of a total of 2,786,687 subscriptions of which496,879 are unique. A Blogline user’s feeds are typ-ically organized into named folders, such as Podcastsor Politics and the folder structure is maintained in theOPML representation. The folder names can be usedas an approximation of the topic that a user associatedwith a feed. By clustering related folders, we can inducean intuitive set of topics for feeds and blogs. Figure 3shows a tag cloud of popular topics aggregated fromreadership information. Finally, we rank the feeds rel-evant to each of the topics generated. In our approach,we say that a feed is topically relevant and authoritativeif many users have cate- gorized it under similar foldernames. For example, Table 1 shows the top politicalblogs ranked using readership-based influence metrics.

An important component in understanding influenceis to detect the sentiment and opinions expressed inblog posts. An aggregated opinion over many usersis a predictor for an interesting trend in a community.Sufficient adoption of this trend could lead to a “tip-ping point” and consequently influence the rest of thecommunity. The BlogVox system (Java et al. 2007a;Martineau et al. 2007; Balijepalli 2007) retrieves opin-ionated blog posts specified by ad hoc queries identi-

1http://engadget.com/2http://dailykos.com/3http://ftm.umbc.edu/4http://bloglines.com/5OPLM, or Outline Processor Markup Language, is an

XML format commonly used to share lists of Web feed URLs.

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Figure 3: The tag cloud generated from the top 200folders before and after merging related folders. Thesize of the word is scaled to indicate how many usersuse the folder name.

1 http://www.talkingpointsmemo.com2 http://www.dailykos.com3 http://atrios.blogspot.com4 http://www.washingtonmonthly.com5 http://www.wonkette.com6 http://instapundit.com7 http://www.juancole.com8 http://powerlineblog.com9 http://americablog.blogspot.com10 http://www.crooksandliars.com

Table 1: The Feeds That Matter for ‘Politics’ shows thetop political blogs ranked using readership-based influ-ence metrics.

fying an entity or topic of interest (e.g., “March of thePenguins”). After retrieving posts relevant to a topicquery, the system processes them to produce a set of in-dependent features estimating the likelihood that a postexpresses an opinion about the topic. These are com-bined using an SVM-based system and integrated withthe relevancy score to rank the results.

Since blog posts are often informally written, poorlystructured, rife with spelling and grammatical errors,and feature non-traditional content they are difficult toprocess with standard language analysis tools. Perform-ing linguistic analysis on blogs is plagued by two ad-ditional problems: (i) the presence of spam blogs andspam comments and (ii) extraneous non-content includ-ing blog rolls, link rolls, advertisements and sidebars.In the next section we describe techniques designed toeliminate spam content from a blog index. This is a vi-tal task before any useful analytics can be supported onsocial media content.

In the following sections we also introduce a tech-nique we call “link polarity”. We represent each edgein the influence graph with a vector of topic and corre-sponding weights indicating either positive or negative

sentiment associated with the link for a Web resource.Thus if a blog A links to a blog B with a negative senti-ment for a topic T, influencing B would have little effecton A. Opinions are also manifested as biases. A com-munity of ipod fanatics, for example, needs little or noconvincing that it is a good product. Thus, attemptingto influencing an opinion leader in such already posi-tively biased communities will have less impact. Us-ing link polarity and trust propagation we have demon-strated how like-minded blogs can be discovered andthe potential of using this technique for more genericproblems such as detecting trustworthy nodes in webgraphs (Kale et al. 2007).

Existing models of influence have considered a staticview of the network. The Blogosphere, on the otherhand, is extremely dynamic and “buzzy”. New topicsemerge and blogs constantly rise and fall in popularity.By considering influence as a temporal phenomenon,we can find key individuals that are early adopters or“buzz generators” for a topic. We propose an abstractmodel of the Blogosphere that provides a systematic ap-proach to modeling the evolution of the link structureand communities. Thus in order to model influence onthe Blogosphere, we need to consider topic, readership,community structure, sentiment and time.

In the following sections, we provide a detailed de-scription of various issues that need to be handled inorder to model influence. Detecting influence and un-derstanding its role in how people perceive and adopt aproduct or service provides a powerful tool for market-ing, advertising and business intelligence. This requiresnew algorithms that build on social network analysis,community detection and opinion extraction.

Detecting blog spamAs with other forms of communication, spam has be-come a serious problem in blogs and social media, bothfor users and for systems that harvest, index and analyzegenerated content. Two forms of spam are common inblogs’: spam blogs (also known as splogs) where theentire blog and hosted posts are machine generated, andspam comments where authentic posts feature machinegenerated comments. Though splogs continue to be aproblem for web search engines and are considered aspecial case of web spam, they present a new set of chal-lenges for blog analytics. Given the context of this pa-per and the intricacies of indexing blogs (Mishne 2007)we limit our discussion to splogs.

Blog search engines index new blog posts by process-ing pings from update ping servers, intermediary sys-tems that aggregate notifications from updated blogs.Scores of spam pages infiltrate at these ping serversincreasing computational requirements, corrupting re-sults, and eventually reducing user satisfaction. We es-timate that more than 50% of all pings are from spamsources (Kolari, Java, & Finin 2006). Two kinds ofspam content sources are prevalent:

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Figure 4: This example of a splog contains plagiarizedcontent(ii), promotes other spam pages by linking tothem (iii) and (i) hosts high paying advertisements au-tomatically selected to match the splog’s content.

• Non-Blogs are pages that attempt to increase the vis-ibility of hosted and linked-to content, by feigning tobe blogs to leverage higher trust and quicker index-ing by web search engines. An example of one suchnon-blog is an Amazon affiliate (third-party vendorof products) book-selling site that pings an updateping server.

• Spam blogs constitute the second kind of spam.These are blogs created using splog creation tools(Finin 2006), and are either fully or partly machinegenerated. Splogs have two often overlapping mo-tives. The first is the creation of blogs containing gib-berish or hijacked content from other blogs and newssources with the sole purpose of hosting profitablecontext based advertisements. The second is the cre-ation of blogs which realize link farms intended toincrease the ranking of affiliate sites (blogs or non-blog web-pages). One such splog is shown in figure4.

Detecting SplogsOver the past year (Kolari 2007) we have developedtechniques to detect spam blogs as they fit the overall ar-chitecture (figure 5), arrived at through our discussionswith practitioners. Our existing and continuing workhas explored all aspects of this architecture. We dis-cuss highlights of our effort based on splog detection us-ing blog home-pages with local and relational features.Interested readers are referred to (Kolari et al. 2006;Kolari, Finin, & Joshi 2006) for further details.

Results reported in the rest of this section are basedon a seed data set of 700 positive (splogs) and 700 neg-ative (authentic blog) labeled examples containing theentire HTML content of each blog home-page. All ofthe models are based on SVMs (Boser, Guyon, & Vap-nik 1992), which are known to perform well in clas-

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Figure 5: In our online spam detection system, pingsfrom existing ping servers are aggregated and passthrough a stream based filtering system. Sub-filters areordered based on their cost of filtering with each decid-ing whether to reject a ping as spam or pass it through.Relational techniques are used in the offline component.The system uses all these detection techniques togetherto adapt and co-evolve through a learning component.

sification tasks (Joachims 1998). We use linear kernelwith top features chosen using mutual information, andmodels evaluated using one-fold cross validation. Weview detection techniques as local and relational, basedon feature types used.

Local FeaturesA blog’s local features can be quite effective for splogdetection. A local feature is one that is completely de-termined by the contents of a single web page, i.e. itdoes not require following links or consulting other datasources. A local model built using only these featurescan provide a quick assessment of the authenticity ofblogs. We have experimented with many such models,and our results are summarized in Figure 6.

(i) Words. To verify their utility, we created bag-of-words for the samples based on their textual content.We also analyzed discriminating features by orderingfeatures based on weights assigned to them by the linearkernel. It turns out that the model was built around fea-tures which the human eye would have typically over-looked. Blogs often contain content that expresses per-sonal opinions, so words like “I”, “We”, “my”, “what”appear commonly on authentic blog posts. To this ef-fect, the bag-of-words model is built on an interesting“blog content genre”. In general, such a content genreis not seen on the Web, which partly explains why spamdetection using local textual content is less effectivethere.

(ii) Word N-Grams. An alternative methodologyto using textual content for classification is the bag-of-word-N-Grams, where N adjacent words are used

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Figure 6: The performance of local models, as mea-sured by the standard, area under the curve metric,varies for different feature types and sizes.

as a feature. We evaluated both bag-of-word-2-Gramsand bag-of-word-3-Grams, which turned out to be al-most as effective as bag-of-words. Interesting discrim-inative features were observed in this experiment. Forinstance, text like “comments-off” (comments are usu-ally turned-off in splogs), “new-york” (a high payingadvertising term), “in-uncategorized” (spammers do notbother to specify categories for blog posts) are featurescommon to splogs, whereas text like “2-comments”, “1-comment”, “i-have”, “to-my” were some features com-mon to authentic blogs. Similar features ranked highlyin the 3-word gram model.

(iii) Tokenized Anchors. Anchor text is the text thatappears in an HTML link (i.e., between the <a...>and </a> tags.) and is a common link-spamming tech-nique around profitable contexts. We used a bag-of-anchors feature, where anchor text on a page, with mul-tiple word anchors split into individual words, is used.Note that anchor text is frequently used for web pageclassification, but typically to classifying the target pagerather than the one hosting the link. We observed that“comment” and “flickr” were among the highly rankedfeatures for authentic blogs.

(iv) Tokenized URLs. Intuitively, both local andoutgoing URLs can be used as effective attributes forsplog detection. This is motivated by the fact that manyURL tokens in splogs are in profitable contexts. Weterm these features as bag-of-urls, arrived at by tokeniz-ing URLs using “/” and “.”. Results indicate this can bea useful approach complementing other techniques.

Relational FeaturesA global model is one that uses some non-local features,i.e., features requiring data beyond the content of Webpage under test. We have investigated the use of linkdistributions to see if splogs can be identified once theyplace themselves on the blog (web) hyper-link graph.The intuition is that that authentic blogs are very un-likely to link to splogs and that splogs frequently dolink to other splogs. We have evaluated this approach

by extending our seed dataset with labeled in-links andout-links, to achieve AUC values of close to 0.85. In-terested readers are referred to (Kolari et al. 2006;Kolari 2007) for further details.

Future ChallengesThough current techniques work well, the problem ofspam detection is an adversarial challenge. In our con-tinuing efforts we are working towards better address-ing concept drift and leveraging community and rela-tional features. The problem of spam in social media isnow extending well beyond blogs and is quite commonin popular social tools like Myspace and Facebook. Thenature of these social tools demand additional emphasison relational techniques, a direction we are exploring aswell.

Recognizing Blogosphere communitiesUnderlying most forms of social media is the concept ofa community of people. Identifying these communties,and their possible sub-communities, continues to be aubiquitous and important task. Most work on commu-nity detection (Gibson, Kleinberg, & Raghavan 1998)is based on the analysis of the networks associated withsocial system.

We have addressed the problem of community de-tection in the Blogosphere by modeling trust and in-fluence (Kale et al. 2007; Kale 2007). Our approachuses the link structure of blog graph to associate senti-ments with the links connecting blogs. Such links aremanifested as URL that blogger a uses in his blog postto refer to blogger b’s post. We call this sentiment aslink polarity and the sign and magnitude of this valueis based on the sentiment of text surrounding the link.These polar edges are evidence of bias/trust/distrust be-tween respective blogs. We then use trust propagationmodels to “spread” the polarity values from a subset ofnodes to all possible pairs of nodes. We have evalu-ated this technique of using trust propagation on po-lar links in the domain of political blogs by predictingthe “like-mindedness” of blogs oriented toward either aDemocratic or Republican position. In order to deter-mine a blog’s bias, we compute its trust/distrust scorefrom a seed set of influential blogs (discussed later) anduse a hand-labeled dataset to validate our results. Moregenerally, we address the problem of detecting all suchnodes that a given node would trust even if it is not di-rectly connected to them.

Link PolarityThe term “link polarity” represents the opinion of thesource blog about the destination blog. In order to deter-mine the sentiment based on links, we analyze sectionof text around the link in the source blog post to deter-mine the sentiment of source blogger about the desti-nation blogger. The text neighboring the link providesdirect meaningful insight into blogger a’s opinion about

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blogger b. Hence, we consider a window of x charac-ters (x is variable parameter for our experimental vali-dations) before and after the link. Note that this set of2x characters does not include html tags.

For our requirements, we do not need to employcomplex natural language processing techniques sincebloggers typically convey their bias about the post/blogpointed by the link in a straightforward manner. Hence,we use a manually created lexicon of positive and nega-tive oriented words and match the token words from theset of 2x characters against this corpus to determine thepolarity. Since bloggers frequently use negation of sen-timental words to indicate bias about another blog-post(“What B says is not bad”), our corpus includes sim-ple bi-gram patterns of the form “not positive/negativeword” .

We adopted the following formula for calculating thelink polarity between two posts:

Polarity = (Np – Nn) / (Np + Nn).Np : Number of positively oriented wordsNn : Number of negatively oriented words

Notice that our formula incorporates zero polaritylinks automatically. The term in the denominator en-sures that the polarity is weighed according to the num-ber of words matched against the lexicon. We use sum-mation as the aggregation technique for computing thepolarity between two blogs. For our experiments, wechoose a domain with a low probability of “off-the-topic” posts within a single blog, hence the notion ofsumming post-post polarity values to yield a blog-blogpolarity value holds.

Trust PropagationSince blog graphs are not always be densely connected,we will not have the trust scores between many pairs ofnodes. Hence, we have investigated techniques for in-fering a trust relationship for nodes where one is not ex-plicitly known. Guha et al. (Guha et al. 2004) have useda framework to spread trust in a network bootstrappedby a known set of trusted nodes. Their approach usesa “belief matrix” to represent the initial set of beliefsin the graph. This matrix is generated through a com-bination of known trust and distrust among a subset ofnodes. This matrix is then iteratively modified by us-ing “atomic propagations”. The “atomic propagation”step incorporates direct propagation, co-citation, trans-pose trust and trust coupling as described in Fig 7. Fi-nally “rounding” technique is applied on the final ma-trix to produce absolute values of trust between all pairof nodes.

In order to form clusters after the step of trust prop-agation, we take the approach of averaging trust scorefor all blog nodes from a predefined set of “influential”nodes belonging to each community. A positive trustscore indicates that the blog node belongs to the com-munity influenced by the trusted node of that commu-nity. Specifically, we selected top three influential (us-ing the number of inlinks as the measure) Democratic

and Republican bloggers. A positive trust score for ablog from top three Democratic blogs indicates that itbelongs to the Democratic cluster and a negative scoreindicates that it is a Republican blogger.

Figure 7: These four graphs represent our atomic prop-agation patterns. The solid and dotted arrows representknown and inferred trust scores, respectively. The first,for example, indicates that if A and C trust B, then Aand C are likely to trust each other.

ExperimentsWe choose political blogs as our domain; one of the ma-jor goals of the experiments was to validate that our pro-posed approach can correctly classify the blogs into twosets: Republican and Democratic. Through some man-ual analysis of the political blogs, we observed that thelink density among political blogs is reasonably highand hence we could deduce the effectiveness of ourapproach by running our algorithms over fairly smallnumber of blogs.

Guha’s work argues that “one step distrust” providesthe best trust propagation results in their domain of ex-periments. They propose the notion of “trust and dis-trust” between two nodes in the graph where the sameset of two nodes can trust or distrust each other. The“one step distrust” approach uses the “trust matrix” asthe belief matrix. However, we believe that in our do-main the initial belief matrix should incorporate bothtrust and distrust (positive and negative polarities fromblog A to blog B). Hence, we use the difference betweenthe trust and distrust matrices as our initial belief matrix.We experimented with various values of the “alpha vec-tor” (the vector used to define the fractional weights foratomic propagation parameters) to confirm that Guha’sconclusion of using the values they proposed {0.4, 0.4,0.1, 0.1} yields best results. Further, Guha et al. recom-mend performing “atomic propagations” approximately20 times to get best results; we took the approach of it-eratively applying atomic propagations till convergenceand our experiments indeed indicate a value close to 20.

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Test Dataset Our test dataset consists of a blog graphcreated from the link structure of Buzzmetrics (Nielsen-Buzzmetric ) dataset. The dataset consists of about 14million weblog posts from three million weblogs col-lected by Nielsen BuzzMetrics for May 2006. The datais annotated with 1.7 million blog-blog links (buz ).

Reference Dataset Adamic and Glance (Adamic &Glance 2005) provided us with a reference dataset of1490 blogs with a label of Democratic and Republicanfor each blog. Their data on political leaning is basedon analysis of blog directories and manual labeling andhas a timeframe of 2004 presidential elections.

Our test dataset from Buzzmetrics did not provide aclassified set of political blogs. Hence, for our exper-iments we used a snapshot of Buzzmetrics that had acomplete overlap with our reference dataset to validatethe classification results. The snapshot contained 297blogs, 1309 blog-blog links and 7052 post-post links.The reference dataset labeled 132 blogs as Republicansand 165 blogs as Democrats (there did not exist anyneutral labels).

Figure 8: Our experiments show that using polar linksfor classification yields better results than plain linkstructure.

Effect of Link Polarity The results in Fig 8 indi-cate a clear improvement on classifying Republican andDemocratic blogs by applying polar weights to linksfollowed by trust propagation. We get a “cold-start” forDemocratic blogs and we observe that the overall re-sults are better for Republican blogs than Democraticblogs. The results being better for Republican blogscan be attributed to the observations from (Adamic &Glance 2005) that Republican blogs typically have ahigher connectivity then Democratic blogs in the politi-cal blogosphere.

We believe that the idea of polar links is quite use-ful and can be applied to multiple domains. The maincontribution of this work lies in applying trust propa-gation models over polar links. We demonstrated onesuch application in the domain of political blogospherewhere we used natural language processing to deducethe link polarity. We would like to emphasize that the

specific techniques to generate polar links is orthogonalto our main contribution and our approach can be eas-ily adapted to different domains for modeling trust anddetecting pre-defined communities.

A generative model for the BlogosphereThe blog analysis techniques described in the earliersections are data-intensive. They require large amountsof blog data that must be obtained by crawling the Blo-gosphere. Moreover, if the collection is not comprehen-sive, attention needs to be paid to ensure a representa-tive sample. Once collected, the data must be cleanedto remove spurious spam blogs, or splogs (Kolari et al.2006) and preprocessed in various ways. (Leskovec etal. 2007; Shi, Tseng, & Adamic 2007). To overcomethe similar difficulty in Web analysis, various graphmodels have been proposed for the structural and statis-tical analysis, including the BA model (Barabasi & Al-bert 1999) and Pennock model (Pennock et al. 2002).However, these models are not suitable for generatingthe blog graphs. While the blog networks resemblemany properties of Web graphs, the dynamic nature ofthe Blogosphere and the evolution of the link structuredue to blog readership and social interactions is not wellexpressed by the existing models.

There are several motivations for developing a gen-erative model for the Blogosphere. First, we hopethat such a model might help us understand variousaspects of the Blogosphere at an abstract level. Sec-ondly, noticing that portion of the blog graph deviatesfrom the Blogosphere graph can signal that somethingis amiss. Spam blogs, for example, often form commu-nities whose structural properties are very unlike thoseof naturally occurring blogs. Third, a generative modelcan be used to create artificial datasets of varying sizesthat simulate portions of the Blogosphere which is oftenuseful for testing and comparing algorithms and sys-tems. For example, testing a model with hidden vari-ables that measures a blog’s influence can benefit fromthe simulated Blogosphere with different blog graphstructures.

Modeling blogger characteristicsOur generative model to construct blog graphs is basedon the general characteristics of the bloggers as ob-served in a cecent PEW Internet Survey (Lenhart & Fox2006) which can be summarized as follows:

• Blog writers are enthusiastic blog readers.• Most bloggers post infrequently.• Blog readership can be inferred through blogrolls, a

list of links to related or friends’ blogs. Active blog-gers are more likely to have a blogroll and follow itregularly.

Our model uses the elements of the existing prefer-ential attachment (Barabasi & Albert 1999) and randomattachment models (Chung & Lu 2006). Each blogger is

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assumed to be reading, writing or being idle accordingto the preferential selection of the bloggers. This helpsto capture the linking pattern arising in the Blogospherethrough local interactions. Local interactions refers tothe interaction of the bloggers among the other blogsthat are generally connected to them either by an inlinkor an outlink. We have studied the properties includingthe degree distributions, degree correlation, clusteringcoefficient, average degree, reciprocity and the distri-bution of connected components. To the best of ourknowledge, there exist no general models to generatethe blog and post networks which possess the propertiesobserved in the real world blogs. Table gives a quickcomparison of the properties of the existing Web mod-els and shows the need for a model for Blogosphere.

Defining blog and post networksOur model of the Blogosphere includes two related net-works, one for blogs and one for their posts. The blognetwork (9a) is defined as a network of blogs obtainedby collapsing all directed post links between blog postsinto directed edges between blogs. Blog networks givea macroscopic view of the blogosphere and help to in-fer a social network structure, under the assumption thatblogs that are “friends” link each other more often. Thepost network (Leskovec et al. 2007) (9b) is formed byignoring the posts’ parent blogs and focus on the linkstructure among posts only. Each post also has a times-tamp of the post associated with it. Post networks givea microscopic view of the blogosphere with details likewhich post linked to which other post and at what time.

Figure 9: The graph representation for the Blogosphereincludes both a blog network and post network.

Design considerationsIn designing our model we relied on our experience inanalyzing, using and generating blog content as well asan investigation into previous models for social mediagraphs. We describe how we addressed some of theseobservations in our model in the following paragraphs.

Linking of new blogs: The new blogger (blog node6)may join the existing network by linking to a popular

6Blog and blogger are synonymous and the exact meaningis evident from the context

blog (a blog node with high in-degree) or may not link atall. Many Web models using continuous growth (Chung& Lu 2006) also use similar techniques for addition ofthe new node.

Linking in blogosphere: Generally active bloggersread several posts and tend to link to some of the poststhat they read recently but the only “observable behav-ior” is the creation of a link to the read post (destina-tion). We model this behavior by having the bloggerkeep track of the recently read blog posts and link tothem.

Linking to a post: Leskovec et al. (Leskovec et al.2007) observed that any post gathers most of its inlinkswithin 24 hours of post time. We approximate this be-havior by linking to recent posts (within a fixed win-dow) of the visited blog when our blogger visits anyblog node.

Blogger neighborhood: Active bloggers tend to sub-scribe to the well known blogs of interest and readthe subscriptions regularly forming blog readership(Lenhart & Fox 2006) (e.g., Bloglines7). Hence we seethat the blogger interactions are largely concentrated inthe blog neighborhood, i.e. nodes connected by eitherinlinks or outlinks.

Use of emerging tools in Blogosphere: New tools inBlogosphere help to discover popular blogs. For ex-ample, blog search engines such as Technorati, socialbookmarking and ranking systems like Del.icio.us andDigg, blog classification systems like Feeds that Mat-ter, and trend discovery systems like BlogPulse. Theavailability and use of these systems mean that blog postreads are not totally random but biased towards the pop-ularity of the blogs which are initially not known to theblogger.

Conversations through comments and trackbacks:The exchange of links among bloggers through com-ments and trackbacks leads to higher reciprocity (i.e.,through reciprocal links) in the Blogosphere than therandom networks. Bloggers tend to link to the blogsto which they have linked in the past either throughcomments, trackbacks or general readership. We ex-pect these local interactions to provide for a higher clus-tering coefficient (as observed in Blogosphere) than therandom networks.

Activity in the Blogosphere: Not all bloggers are “ac-tive” (either reading or writing) at all times. Only asmall portion of Blogosphere is active with the remain-der identidied as idle. This activity can be approximatedby observing the number of links created within a timespan. We use a super linear growth function to modelthe activity as defined by Leskovec et al. (Leskovec,Kleinberg, & Faloutsos 2007). The outlinks from ablog can be considered as the measure of an active blogwriter because an active writer will naturally look formore interesting sources to link to. The reverse maynot be true that the blogger who reads a lot also writes

7http://www.bloglines.com

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Property ER model BA model Blogosphere Our SimulationType undirected undirected directed directedDegree distribution poisson power law power law power lawSlope [inlinks,outlinks] N/A [2.08,-] [1.66-1.8,1.6-1.75] [1.7-2.1,1.5-1.6]Avg. degree constant (for given p) constant (adds m edges) increases increasesComponent distribution N/A (undirected) N/A (undirected) Power law Power lawCorrelation coefficient - 1 (fully preferential) 0.024 (WWE) 0.1Avg clustering coeff. 0.00017 0.00018 0.0235 (WWE) 0.0242Reciprocity N/A (undirected) N/A (undirected) 0.6 (WWE) 0.6

Table 2: This table shows various graph properties for two popular network models (ER and BA), an empirical studentof a Blogosphere sample, and our simulated model.

more.

Experiments and ResultsThe preferential attachment model as proposed byBarabasi (Barabasi & Albert 1999) obtains the powerlaw degree distributions in an undirected network.However, this model is not defined for a directed net-work. The model proposed by Pennock et al. (Pen-nock et al. 2002) captures the random behavior butdoes not capture the local interactions among nodes inthe graph. We use the “alpha preferential attachment”model proposed by Chung et al. (Chung & Lu 2006)to obtain power law degree distributions in a directedgraph. We have modified this model to reflect local in-teraction among the bloggers by using preferential at-tachment among neighboring nodes. The details of ouralgorithm can be found in (Karandikar 2007).

Part of our evaluation is done by comparing the dis-tinguishing properties of the real blog graphs with theresults of our simulation. These properties were verifiedagainst two large blog datasets available for researchersnamely WWE8 2006 and ICWSM9 2007.

Figure 10 shows the power law curve for inlink distri-bution observed in the blog network of the “simulated”blogosphere. Similarly, figure 11 compares the distri-bution of the strongly connected components (SCC) inthe simulation and the blogosphere.

Figure 12 shows the scatter plot for in-degree andout-degree correlations in the simulated Blogosphere.The plot shows the low degree correlation as observedin the real Blogosphere by Leskovec et al (Leskovec etal. 2007).

Being able to generate synthetic blog data is usefulfor testing and evaluating algorithms for analyzing andextracting information from the Blogosphere. The util-ity, however, depends on the generated graphs beingsimilar to the actual ones in key properties. We havecreated a model that accounts for several key featuresthat influence how the Blogosphere grows and its result-

8a dataset developed for the Workshop on the WebloggingEcosystem held at the 2006 World Wide Web Conference

9International Conference on Weblogs and Social Media:http://www.icwsm.org/data.html

Blog network ICWSM WWE SimulationTotal blogs 159,036 650,660 650,000

Blog-blog links 435,675 1,893,187 1,451,069Unique links 245,840 648,566 1,158,803

Average degree 5.47 5.73 4.47Indegree distribution -2.07 -2.0 -1.71

Outdegree distribution -1.51 -1.6 -1.76Degree correlation 0.056 0.002 0.10

Diameter 14 12 6Largest WCC size 96,806 263,515 617,044Largest SCC size 4,787 4,614 72,303

Clustering coefficients 0.04429 0.0235 0.0242Percent Reciprocity 3.03 0.6838 0.6902

Table 3: This table compares a simulated blog networkwith two datasets based on blogs harvested from theWeb using a number of standard graph metrics.

ing structure. By selecting appropriate parameters forour model, we can generate graphs that more closelyapproximate observed blogosphere network propertiesthan previous network models.

ConclusionSocial media systems are increasingly import on theWeb and account for a significant fraction of new con-tent. The various kinds of social media are alike inthat they all have rich underlying network structuresthat provide metadata and context that can help whenextracting information from their content. We havedescribed some initial results from ongoing work thatis focused on extracting, modeling and exploiting thisstructural information from the underlying networks.

As the Web continues to evolve, we expect that theways people interact with it, as content consumers aswell as content providers, will also change. The re-sult, however, will continue to represent an interestingand extravagant mixture of underlying networks – net-works of individuals, groups, documents, opinions, be-liefs, advertisements, and scams. These interwoven net-works present new opportunities and challenges for ex-tracting information and knowledge from them.

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Post network ICWSM WWE SimulationTotal posts 1,035,3611,527,348 1,380,341

Post-post links 1,354,6101,863,979 1,451,069Unique links 458,950 1,195,072 1,442,525

Avg post outlinks 1.30 1.22 1.051Average degree 2.62 2.44 2.10

Indegree distribution -1.26 -2.6 -2.54Outdegree distribution -1.03 -2.04 -2.04

Degree correlation -0.113 -0.035 -0.006Diameter 20 24 12

Largest WCC size 134,883 262,919 1,068,755Largest SCC size 14 13 3

Clustering coefficients 0.0026 0.00135 0.00011Percent Reciprocity 0.029 0.021 0.01

Table 4: Comparison of post network properties ofdatasets and simulation

Figure 10: Simulation: Blog inlinks distribution

AcknowledgementsWe thank Dr. James Mayfield, Justin Martineau andSandeep Balijepalli for their contributions to the workon sentiment detection. Partial support for this re-search was provided by the National Science Foun-dation (awards ITR-IIS-0325172 and NSF-ITR-IDM-0219649) and I.B.M.

ReferencesAdamic, L. A., and Glance, N. 2005. The politicalblogosphere and the 2004 U.S. election: divided theyblog. In LinkKDD ’05: Proceedings of the 3rd interna-tional workshop on Link discovery, 36–43. New York,NY, USA: ACM Press.Balijepalli, S. 2007. Blogvox2: A modular domain in-dependent sentiment analysis system. Master’s thesis,University of Maryland, Baltimore County.Barabasi, A.-L., and Albert, R. 1999. Emergence ofscaling in random networks. Science 286:509.Boser, B. E.; Guyon, I. M.; and Vapnik, V. N. 1992.

Figure 11: Distribution of SCC in blog network (Simu-lation and Blogosphere)

Figure 12: Simulated blog network scatter plot: Outde-gree vs. Indegree

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2004. Propagation of trust and distrust. In WWW’04: Proceedings of the 13th international conferenceon World Wide Web, 403–412. New York, NY, USA:ACM Press.Java, A.; Kolari, P.; Finin, T.; Joshi, A.; Martineau,J.; and Mayfield, J. 2007a. The BlogVox OpinionRetrieval System. In Proceedings of the Fifteenth TextREtrieval Conference (TREC 2006).Java, A.; Kolari, P.; Finin, T.; Joshi, A.; and Oates, T.2007b. Feeds That Matter: A Study of Bloglines Sub-scriptions. In Proceedings of the International Con-ference on Weblogs and Social Media (ICWSM 2007).Computer Science and Electrical Engineering. To Ap-pear.Joachims, T. 1998. Text categorization with supportvector machines: Learning with many relevant fea-tures. In ECML ’98: Proceedings of the 10th Eu-ropean Conference on Machine Learning, 137–142.London, UK: Springer-Verlag.Kale, A.; Karandikar, A.; Kolari, P.; Java, A.; Joshi,A.; and Finin, T. 2007. Modeling Trust and Influencein the Blogosphere Using Link Polarity. In Proceed-ings of the International Conference on Weblogs andSocial Media (ICWSM 2007). Short Paper.Kale, A. 2007. Modeling trust and influence in blogo-sphere using link polarity. Master’s thesis, Universityof Maryland, Baltimore County.Karandikar, A. 2007. Generative Model To ConstructBlog and Post Networks In Blogosphere. Master’s the-sis, University of Maryland, Baltimore County.Kempe, D.; Kleinberg, J. M.; and Tardos, E. 2003.Maximizing the spread of influence through a socialnetwork. In proceedings of the Ninth ACM SIGKDDInternational Conference on Knowledge Discoveryand Data Mining, 137–146.Kempe, D.; Kleinberg, J. M.; and Tardos, E. 2005.Influential nodes in a diffusion model for social net-works. In Proceedings of the 32nd International Col-loquium on Automata, Languages and Programming,1127–1138.Kolari, P.; Java, A.; Finin, T.; Oates, T.; and Joshi,A. 2006. Detecting Spam blogs: A machine learningapproach. In Proceedings of the 21st National Confer-ence on Artificial Intelligence. AAAI Press.Kolari, P.; Finin, T.; and Joshi, A. 2006. SVMs forthe blogosphere: Blog identification and splog detec-tion. In AAAI Spring Symposium on ComputationalApproaches to Analyzing Weblogs. AAAI Press.Kolari, P.; Java, A.; and Finin, T. 2006. Character-izing the splogosphere. In WWW 2006, 3rd AnnualWorkshop on the Webloggging Ecosystem: Aggrega-tion, Analysis and Dynamics.Kolari, P. 2007. Detecting Spam Blogs: An AdaptiveOnline Approach. Ph.D. Dissertation, University ofMaryland, Baltimore County.

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Biographical sketchesTim Finin is a Professor of Computer

Science and Electrical Engineering at theUniversity of Maryland Baltimore County(UMBC). He has over 35 years of expe-rience in the applications of AI to prob-lems in information systems, intelligent in-terfaces and robotics. He holds degrees from MITand the University of Illinois and has held positions atUnisys, the University of Pennsylvania, and the MIT AILaboratory.

Anupam Joshi is a UMBC Professor withresearch interests in the broad area of net-worked computing and intelligent systems.He currently serves on the editorial boardof the International Journal of the SemanticWeb and Information.

Pranam Kolari is a member of the tech-nical staff at Yahoo! Applied Research. Hereceived a Ph.D. in Computer Science. Hisdissertation was focused on spam blog de-tection, with tools developed in use both byacademia and industry. He has active re-search interest in internal corporate blogs, the SemanticWeb and blog analytics.

Akshay Java is a UMBC PhD student.His dissertation is on identifying influenceand opinions in social media. His researchinterests include blog analytics, informa-tion retrieval, natural language processingand the Semantic Web.

Anubhav Kale received a M.S. degreein Computer Science from UMBC in May2007. His thesis research demonstrated theeffectiveness of detecting sentiment associ-ated with links between blog posts and us-ing this to enhance blog community recog-nition algorithms. He is currently a software engineerat Microsoft.

Amit Karandikar received a M.S. degreein Computer Science from UMBC in May2007. His thesis research produced a gen-erative model for the blogosphere whichmodeled both the reading and writing activ-ities of bloggers. He is currently a softwareengineer at Microsoft.

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