Recently, much work has been done to investigate the structure and dynamics of large social networks. Through datasets from online social networking sites, computer scientists are given an unprecedented look into the networks many of us form everyday. One of the key research questions that has arisen is about the growth of online communities. How do groups form online? How does the number of a given person’s friends who have already joined a group influence his decision? What about the community’s social density? Is a person’s decision influenced by the network structure of his group of friends who have joined the group? What about the structure of the group as a whole? By examining the development of online communities and seeking to answer such questions, sociologists can infer a great deal about how humans interact, whether online or off.
In fact, such issues do play important roles in the development of online communities. For the sake of this blog post, however, I intend to address just one: the trend of “triadic implosion” in the growth of online communities. In “Group Formation in Large Scale Social Networks: Membership, Growth, and Evolution”, the authors use LiveJournal to examine these network phenomenon. One interesting (and someone counter-intuitive) finding of the paper was that growth increased with the ratio of closer to open triads only to a certain value, at which point it dropped sharply.
But before I explain the sudden drop in growth, let me elucidate the logic behind the initial correlation. Based on the concept of weak ties, one could say that there is a trust advantage to joining a community in which your friends know each other. Specifically, a new community member would theoretically be supported by a richer social structure in such a community. Another point, consistent with the findings of “Cascade Dynamics of Multiplex Propagation”, is that successful social diffusion commonly occurs in “highly clustered networks” since a “coordination effect” may come into play, in which a non-member receives a greater net endorsement of a community if there is a “shared focus of interest among a group of interconnected friends.”
Once the closed to open triad ratio hits 0.15, however, there is a tipping point, and the rate of growth drops suddenly. This phenomenon has been colloquially dubbed “triadic implosion.” There are a couple of hypotheses for this phenomenon. The first is that the high ratio of closed to open triads implies a sense of “cliqueishness.” In other words, the community is so tightly connected that it is either too hard to break into, or becomes undesirable for potential members. Another possible explanation is that the high density of the group implies that it has reached a stage when it has stopped gaining new members and has subsequently begun densifying, (i.e. forming edges between its current set of nodes). Both of these hypotheses beg further research and would make interesting projects for continued work.
L. Backstrom, D. Huttenlocher, J. Kleinberg, X. Lan.
Group Formation in Large Social Networks: Membership, Growth, and Evolution.
Proc. 12th ACM SIGKDD Intl. Conf. on Knowledge Discovery and Data Mining, 2006
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