document: Group formation in large social networks : membership , growth , and evolution The processes by which communities come together , attract new members , and develop over time is a central research issue in the social sciences - political movements , professional organizations , and religious denominations all provide fundamental examples of such communities . In the digital domain , on-line groups are becoming increasingly prominent due to the growth of community and social networking sites such as MySpace and LiveJournal . However , the challenge of collecting and analyzing large-scale time-resolved data on social groups and communities has left most basic questions about the evolution of such groups largely unresolved : what are the structural features that influence whether individuals will join communities , which communities will grow rapidly , and how do the overlaps among pairs of communities change over time . Here we address these questions using two large sources of data : friendship links and community membership on LiveJournal , and co-authorship and conference publications in DBLP . Both of these datasets provide explicit user-defined communities , where conferences serve as proxies for communities in DBLP . We study how the evolution of these communities relates to properties such as the structure of the underlying social networks . We find that the propensity of individuals to join communities , and of communities to grow rapidly , depends in subtle ways on the underlying network structure . For example , the tendency of an individual to join a community is influenced not just by the number of friends he or she has within the community , but also crucially by how those friends are connected to one another . We use decision-tree techniques to identify the most significant structural determinants of these properties . We also develop a novel methodology for measuring movement of individuals between communities , and show how such movements are closely aligned with changes in the topics of interest within the communities .
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