In this paper we analyze the community of a social network site, Advogato. The peculiar characteristics of Advogato is that users can explicitly express weighted trust relationships among themselves. We conduct a longitudinal analysis of the trust network over a time period of 4 years, exploring the community as it grew from a knit circle of 300 users to an society of almost 6500 individuals. We report the changes over time of standard indexes in social network analysis such as clustering and degrees of separation. We then focus on specific measures about trust such as reciprocity and changes over time of average trust. A decline in trust is observed as the community grows. Following what we believe to be the first empirical analysis of trust evolution over time in a real community, we conclude suggesting how the availability of data about human relationships in social network sites is opening up the possibility of monitoring changes in trust in real time. In order to foster this research line, we released the datasets and the code we used in our analysis.

6. Trust statements expressed by raph

17. What I'm going to show you? A measure (y axis)‏ over time (x axis)‏ So its evolution in time Up to 2004 -10-28 Since 2000 -02-25 All datasets and Python code released on www.trustlet.org (Test your hypos!)‏

18. Number of users on Advogato 300 users (in 2000-02-25)‏ 6482 users (in 2004-10-28)‏ Evolution from a close-knit circle (first users knew the funders and each other)‏ to a mature society (new users joined it via web)‏

19. Number of trust edges on Advogato 2109 trust edges (in 2000-02-25)‏ 47943 trust edges (in 2004-10-28)‏ As expected, #edges, just as #users, grows

20. Mean outdegree (outgoing trust statements per user)‏ Surprised? It's a REAL social network! Important to study REAL social network and not synthetized ones because of un- expected activity patterns (1) 2000-02-25: 14.0 (300 users)‏ (2) 2000-07-18: 16.7 (1454 users)‏ what did the 1154 new users did in 5 months? No datasets ;( (3) 2000-08-11: 15.1 (1880 users)‏

21. % users in the main strongly connected component (set in which everyone is reachable by everyone)‏ Decline from 62% down to 48% In the ol' days, everyone knew each other, then, new unknown users start to join (smaller subcommunities)‏ ...

22. Clustering coefficient Same pattern: decline! The network becomes more spread over time

23. Mean degrees of separation (stabilizes over 3.5)‏ Not SIX??? 6 degrees of separation is more a buzzword than reality

24. Percentage of trust statements that get reciprocated In Advogato, it stabilizes ~36% In Flickr, ~66% Facebook, 100% by design (depends on the socio-technological system)‏

25. Reciprocation matrix: what (row) reciprocated how (column)‏

27. % trust values on edges

28. Average trust in the community as expressed by Advogato users over time Trust is declining as society grows. A normal pattern common to every society?

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