I don't twitter.
The only reason I ended up on Facebook is because Ben invited me. I'm still not really sure what to do with it, but I play along and pretend like I do - sort of like I do in most meat space social situations. One of these days I'm going back to Urk-Glar where such things aren't needed.
I think these social tools all provide us with something that we wanted, or needed, in the "1.0" web. Context, personality, etc.,
When the web was created everyone was anonymous, that's less true today thanks to social tools. There was a need to know who you were dealing with, and people had a need/desire/urge to share knowledge and experience. As we shared more, it became more important to know who was doing the sharing, and whether that person could be trusted, and off you go. Facebook and similar sites were born out of this need for context and identity. Thanks to "social networking" you now have an identity on the web, for better or worse.
Twitter, on the other hand, is not of this ilk. Twitter is about speed and spontaneity. Twitter's hayday was at a massive conference where people began using it to talk about, and review, and share, about things that were going on RIGHT NOW. What's the best way to do that? On your cell phone of course! And so twitter set a limit on your messages to keep them text message friendly. Twitter is all about "what's going on right now." more than any kind of identity issue it's trying to solve. Twitter has also become a right-popular publishing tool for snippets and headlines, and so forth. Often times news will break on twitter several minutes before major news organizations can grab, print, and post anything about the event.
So, ultimately, you get two kinds of major tools, I suppose. Publishing tools and identity networks. They work hand in hand to form the "social web" experience. One fosters the other, having an identity makes us easier to trust and having the publishing tools lets us do something with that trust.
OF course it's worth noting that facebook is also fast becoming a communications tool as much as anything else. Communications or "messaging" is probably going to the next major frontier for "Web 2.0"