I never joined Twitter, but wasn't there a pretty great statistics Twitter at some point? Dudes like Larry Wasserman, Nate Silver, Hadley Wickham, Andrew Gelman all pretty active and collaborating in the open. I got the impression it had some nice topical communities like that, whereas the frontpage current events feed was pretty much always shit, but that seems like a universal truth of human commons, not something specific to web-based social media or Twitter.