I haven't gotten the kids to begin tweeting each other for homework help yet (they still universally prefer to go over the tougher problems together in class), but Twitter is actually starting to pay some dividends for collaborative projects.
For example, we're going to be launching model rockets to study force, gas laws, resonance (check out the really cool video of fin flutter embedded above), and aerodynamics. We spent some time in the computer lab researching fuel types, fin designs, and deciding whether we would make our own or use kits. Most of the students were already following me, so they clicked around and began following each other. In doing so, there was obviously a bit of side conversation, but most started posting links, thoughts, and ideas.
We're launching next week - we'll be tweeting the whole thing. This would be a disaster with the informal geometry class I described yesterday, but I do believe that my sharper students are starting to embrace the actual utility that social media might be able to provide. Obviously, there are settings where this has potential; I'll stick with the whiteboard for homework.