Social Networks of Common Interests

The proliferation of social networking sites all share one characteristic in common. Their members share a single common interest, whether it be music, movies, relationships, pornography, politics, gossip, science, etc. There is an equality or value-free hierarchy of common interests  in the market of social networking sites. Nothing is more important, or more base than any other interests. 

What is the important factor that distinguish one successful social networking site, from a dismal failure? Is it the kind of common interest among its members? Is it the tools it provides the members to share? Is it the type of members that it attracts? Is there a singular "Most Important" factor that determines the success or chances for success? Is there a matrix of factors?