Social networks as platforms
I think the main hurdle to creating a successful social network is thinking of these networks as websites or web applications. The real networks exist offline and the web apps are a way of organising or augmenting those relationships.
At first I thought that the success of a network depended on how much it acted as a site-independent platform. You can see with Facebook and Twitter in particular that they have a useful API that has encouraged a ecosystem of applications and tools to flourish around them. New challengers have to compete, not only with the application itself, but the way it has embedded into the web.
But now, I’m starting to think that the distinction between Facebook as a website and Facebook as a platform is a natural extension of the fact that these sites must mirror offline relationships to be successful. If I have a close circle of friends, I don’t have to leave them behind in one place and then reassemble them in another. The tools exist (phone, IM, email) to communicate at all times, around any subject.
We don’t need a new social network for every conceivable topic because Facebook already fills that need. But conversely, we ultimately don’t need Facebook because even without it, our relationships still exist.