Molly Wood has an article on CNET detailing Five reasons social networking doesn't work. I think it's worth reading. Her top reason:
There's nothing to do there.
Amen. The real problem is that once you bother going to a social networking networking website, there is nothing to actually do except email and posting, and we already have perfectly good ways of doing that without them.
It's also interesting to note how her opinions may differ from those of men. There is a definite undercurrent of sexual harrassment that should not be ignored.
Addendum: I just tried to log into orkut for the first time in months. I suddenly remembered why I stopped using it. Within 30 seconds I got an error message indicating that there server had "done something unexpected". Uh, yeah. Bite me.