I think social networking has a viable market waiting to be launched. Imagine Facebook being Web 1.0. I think Facebook 2.0 is out there waiting to be hatched.
Facebook in particular, the largest player in the social networking arena by far, has a huge opportunity before their eyes. Wait for them to start executing on them. I "only" see 2 big ones: advertising and search. Yeah, I know, many people are talking about it. But that's true. FB knows everything about most of their users (600mm people), so how long until they create a competing Adsense service? Not far I think. That's a $10bn a year, at least, for FB in the next 5 years or less. Search is another opportunity that's so huge I can't even think of the size of that market.
I don't know if it would be $10B though, even AdSense isn't quite there for Google yet. Assuming they could lure a big chunk of AdSense publishers away, I suppose it could be several billion though.
Maybe they'll release it just before IPO to show their true promise. Name predictions... FB AdConnect?