I watched a Noam Chomsky youtube video recently, where he said something similar about TV news. The amount of time you get is only about enough to throw out some half-baked soundbite i.e. "XXX are terrorists", "All benefit claimaints are scroungers" and if it doesn't reiterate whatever the perceived wisdom of the day is, there's no way you'll get enough time to make a point.

I don't use twitter but i imagine the 140 character limit must be much worse. Facebook might be generally vacuous but at least there's enough space and persistance of thread that it's possible to have something of an intellectual discussion.