Mark Zuckerberg (Founder & CEO of Facebook) has some interesting thoughts on why his social networking site succeeded where others have failed:
[Mark] thinks that [other sites] have not focused on providing a set of utilities to their audience, they were merely about creating connections.Exactly. Social networking sites like Orkut or Friendster have no purpose. Sure, it's fun. You go there and, in a flurry of activity, set up your profile and list all your friends. It's always good for a little ego pump.
But, then what is there to do with your social network? There's no purpose, no reason to come back, nothing to do.
Social networks should be a tool to solve some other problem. The idea should be, "Now that I have listed all my friends, I can come back to do incredibly cool thing X everyday." That's why sites like Facebook succeed where so many others have failed.