thestructureofonlinesocial$ networks$mirrors$those$in$the$ …ravi/pdfs/talk_slides.d/chenna... ·...
TRANSCRIPT
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The structure of online social networks mirrors those in the
offline world
Paper by R.I.M. Dunbar, Valerio Arnaboldi, Marco Con9, Andrea Passarella
Presenta9on: Rohith Raj Chenna
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Egocentric Social Network Analysis
• Studies an individual’s personal network and its affects on that individual • The Ego: Describes the network around a single node. • Alter: The nodes that are connected to the ego.
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Social Networks and it’s constraints
• There is huge growth in the usage of social networking sites over the past decade. • This has raised fundamental ques9ons about the constraints that exist over both the size and the paMern of social rela9onships and whether they mirror the offline social networks. • This becomes of par9cular interest in the light of the finding that there appears to be a cogni9ve limit on the size of natural face-‐to-‐face social networks. • This limit is thought to arise out of a combina9on of a cogni9ve constraint and a 9me constraint.
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Cogni>ve Constraint
• The central cogni9ve constraint is based on the observa9on that, in primates, the typical size of social groups correlates closely with the size of the neocortex. • If the limit is exceeded, then the social network becomes unstable and is prone to fission. • This proposal is supported by evidence from neuroimaging performed on humans and monkeys.
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Time Constraint
• There is also evidence to suggest that 9me imposes a constrain • Time becomes important because it seems that the strength of a rela9onship is determined by how much 9me two individuals spend together. • Self-‐rated es9mates of the emo9onal closeness for dyadic rela9onships correlate closely with the frequency of contact. • And these in turn correlate with willingness to behave altruis9cally towards the alter in ques9on
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Determina>on of the size of offline Social Networks • Offline social network of heavy and casual users of internet social networking sites, and found that they did not differ. • Study on downloaded traffic among the followers of individual TwiMer accounts and, using a criterion of reciprocated exchanges to iden9fy meaningful rela9onships, concluded that TwiMer communi9es typically averaged between 100 and 200 individuals. • In a physicists community, it was found that that there was a marked downturn in the rate at which addi9onal members were acquired once communi9es exceeded 200 individuals
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• Individuals do not, however, distribute their social effort evenly among the alters in their networks. • Indeed, there is considerable evidence to show that, within natural social networks, individual alters can be ranked in order of declining investment by ego. • These rankings fall into a natural series of layers with a scaling ra9o of ∼3 that yields breakpoints at around 5, 15, 50 and 150 alters.
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Facebook Dataset #1
• Facebook dataset #1 was obtained before 2009 when the default privacy se[ngs allowed users inside the same regional network to have full access to each others’ personal data. • The dataset covers the 9me span from the start of Facebook in September 2004 un9l April 2008. • The dataset represents only a subsample of the original Facebook regional network, in terms of downloaded Facebook profiles (∼56%) and their Facebook friendships (∼37%).
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• Despite the high number of missing profiles, some of their data is s9ll present in the dataset. • We only miss the data that is sent from a public profile to a private profile and between two private profiles. • We do not know which profiles are public and which are not because we only have number of (undirected) interac9ons (posts or photo comments) that occurred. • The only informa9on we have is the percentage of non-‐public profiles
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• 44% of the nodes are selected randomly and are assumed to be private. • The number of interac9ons on all these nodes are doubled. • The resul9ng interac9ons are more accurate in inside layers and less accurate in outside layers. • Strongly asymmetrical rela9onships are typically known to belong to the most external layers of the ego networks.
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• Facebook dataset #1 consists of 3+ million nodes and 23+ million edges. • The data is classified into 4 9me frames. • Last month, Last 6 months, Last year, En9re dura9on. • Contact frequency is calculated.
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CCDF of contact frequency for rela>onships in Facebook dataset #1
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• Graph shows that the contact frequency is low for most of the rela9onships, but there are a few rela9onships with very high levels of interac9ons. • This type of distribu9on is typical in social networks.
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• For the analysis we consider only egos with an average of more than 10 interac9ons per month, thus selec9ng “socially ac9ve people” since they are par9cularly relevanior our analysis, and discard inac9ve profiles. • The final dataset contain 130,338 egos with 5,289,910 ac9ve edges. • To extract ego networks from the datasets, we first create a series of sets each of which contains all the social rela9onships of a user.
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CCDF of the size of ego networks for rela>onships in Facebook Dataset #1
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• We note that, for the majority of ego networks, the size is lower than 100. • This means that even though people can poten9ally add up to 5000 friends in Facebook, they communicate only with a small subset of them.
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• We further refine the dataset by selec9ng, for each ego network, only the set of rela9onships with contact frequency higher than one message per year. • This is to avoid considering people in whom the ego does not invest some minimum amount of 9me and cogni9ve resources. • The selec9on of social rela9onships with more than one message per year does not represent a substan9al change in the size of the ego networks.
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Facebook Dataset #2
• It represents the Facebook regional network of New Orleans and it has been obtained through a crawling agent similar to the one created for downloading Facebook dataset #1. • It represents a much smaller network((90,269 nodes and 3,646,662 social links) but the data is much more specific. • It reports the list of its Facebook friends and the list of wall posts received by the user from their friends, with the 9mestamp indica9ng the 9me at which the interac9on occurred.
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TwiIer Dataset
• A sample of 303,902 TwiMer user profiles are collected by crawling TwiMer in November 2012. • To avoid including TwiMer users that are not human, the data is filtered by iden9fying user profiles that have recognizable human characteris9cs.
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CCDF of the contact frequency for rela>onship in TwiIer
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CCDF of ther size of ego networks in TwiIer
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• As with the Facebook datasets, we selected only accounts that had an average of more than 10 interac9ons per month. • The final dataset contains 60,790 egos and 5,323,195 social rela9onships. • The CCDFs show longer tails than in Facebook. This indicates tha9n TwiMer there are users with larger ego networks than in Facebook. • Nevertheless, similarly to Facebook, more than 90% of the TwiMer users have less than 100 rela9onships
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Analysis
• We use both K-‐Means(par99on clustering technique) and DBSCAN(Density based clustering technique) on the frequency of contact of each ego network to search for a layered structure. • We apply k-‐means in two different ways. • On the one hand, we want to find the typical number of clusters in the ego networks, as we want to verify if Facebook and TwiMer ego networks show a layered structure with a number of layers similar to the one found in offline social networks.
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• To measure how well the data are clustered, we calculate the silhoueMe sta9s9cs for each op9mal configura9on. • The results obtained with k-‐means may be affected by the presence of noisy data. • Noise canaffect a k-‐means analysis in two different ways: • The presence of noisy points between two adjacent clusters might cause the algorithm to treat them all as a single cluster instead of two. • The presence of a large number of noisy points in the data set could lead to the detec9on of more clusters than really exist.
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• DBSCAN defines two parameters, ε and MinPts. • Any object with more than MinPts neighbours within a distance ε is defined as a “Core object”. • “Border objects” of the cluster is linked to a core object at a distance less than ε.
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Op>mal number of clusters
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Results
• The distribu9ons show a marked peak around k* = 4 for all the datasets. • For Facebook dataset #1, the ego networks have an average op9mal number of clusters equal to 4.35 (with median 4), and Facebook dataset #2 has an average op9mal number of clusters of 4.10 (with median 4). • . Despite a clear mode at 4, the ego networks in the TwiMer dataset have an average op9mal number of clusters equal to 6.60 (with median 5) due to the long tail to the right.
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• The average silhoueMe value for the best configura9ons associated with the op9mal number of clusters for each ego network is 0.670 for Facebook dataset #1, 0.678 for Facebook dataset #2, and 0.674 for TwiMer
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Layer 0
• We only knew about the occurrence from Layer 1 of approximately 5 friends but we see a new layer before layer 1 called layer 0. • This means most people have 1 or 2 really good friends with whom they communicate way more than that they do with people who are in layer 1.
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Conclusion
• The analyses of three different online datasets confirm the layered structure found in offline face-‐to-‐face social networks. • These layers have previously been iden9fied only from samples of quite modest size. • The sizes of the en9re ego networks for the three datasets are smaller than the total size of conven9onal offline egocentric networks. • The mean rates of contact in each layer are extremely close, especially for the Facebook datasets, to those found in offline egocentric networks. • This suggests that the online environments may be mapping quite closely onto everyday offline networks