It seems to me that Google is right on time: the time in the sun for social networks seems to be about up.
Call it a land rush; call it a bubble; call it a craze. The social networks like FaceBook, MySpace, and the social network apps like Digg enjoyed a moment in the sun as the fleshed out one dimension of the webbernet that hadn't really been fully articulated. Now that it has, you're seeing a lot of the ideas articulated by those sites rolled into more mature, more complex, and more interesting sites and services.
Of course, community for the sake of community was something I always thought was best done face to face, sitting next to someone on a barstool or at a coffee shop. Me, if I am going to look for community on the web, which is really more like what we used to call "association" (that is, a gathering of like-minded individuals), I'm going to look for sites that possess the traits I'm interested in. Like SlashDot or ArsTechnica et cetera.