This is something i've thought about quite a lot. On one hand, the limiations on what user's can do within 140 characters make much of the exchanges on twitter trivial. And for a "social" web site, the amount of true interaction between users is actually very minimal - more like many people in the same roon taking at the same time, hoping someone will listen. I think there is an interesting side effect of this that helps twitter remain popular - that people don't feel personally invested in any of their tweets, so they don't get dejected when most of them go completely ignored.