You are a tree falling and no one is around to hear it. There is a lack of interaction unless you're a celebrity, have outside friends on there, or spent some effort in gaining followers. They need to find ways to break users into small groups who can interact with each other (pretty much the entire appeal of IRC).
I can't figure out a good way to use Twitter. Follow celebrities? Seems cool at first but as much as I like Steven Fry's work, his hourly musings don't add much to my life. Coordinate with friends? Why wouldn't I just text? Keep up with news in areas that interest me? Hacker news and Reddit cover they pretty soundly, and it's much easier to read and participate in discussion. Perhaps professional networking is where it would shine, but emails and mailing lists feel like a better fit to me.
I keep an account because I hear it's good for building that "personal brand" that I think I'm supposed to build at some point.