I signed up for twitter on my phone, purely so I could follow the NBA news, since it gets tweeted before anything else. I have since added a few other people (like Adam Carolla, since he gives info about his podcast, and Conan O'Brien...because he's funny), but I have never had the slightest interest in tweeting anything.
I actually think it is a decent tool to get information out quickly, and to follow that information (although it can be tough to wade through at times). Beyond that, I just don't have a ton of interest. Same goes for facebook "newsfeed"...I really don't care what your doing, and I certainly don't think what I am doing is any of your business.