It is instructive to learn why some social networks proper, and others tank. Social network pioneer Friendster was killed by Bellman's curse of dimensionality, with help of businessmen. Link, but don't open with Safari.
Apparently the algorithms didn't scale well. The businesspeople invested in more hardware and more expensive marketing instead of fixing the technical problem. MySpace took over, until News Corp destroyed it. Its story is less interesting; Murdoch wanted to make money fast. Now the race is between Facebook, Twitter, and G+.