  Although recently there are extensive research on the collaborative networks
and online communities, there is very limited knowledge about the actual
evolution of the online social networks (OSN). In the Letter, we study the
structural evolution of a large online virtual community. We find that the
scale growth of the OSN shows non-trivial S shape which may provide a proper
exemplification for Bass diffusion model. We reveal that the evolutions of many
network properties, such as density, clustering, heterogeneity and modularity,
show non-monotone feature, and shrink phenomenon occurs for the path length and
diameter of the network. Furthermore, the OSN underwent a transition from
degree assortativity characteristic of collaborative networks to degree
disassortativity characteristic of many OSNs. Our study has revealed the
evolutionary pattern of interpersonal interactions in a specific population and
provided a valuable platform for theoretical modeling and further analysis.
