From where I'm sitting, Twitter is doing so much more damage than Facebook that it's not even close.
My Facebook is full of old friends from school not saying much, and some local groups moaning about local goings on.
Twitter, on the other hand, is an absolute and total perversion of human communication. An arbitrary point-scoring game with a tiny character limit, dominated by activists whose skills at the game are carrying their piss-poor arguments. I've got a friend who obviously gets her opinions from Twitter and then can't formulate coherent sentences in real world conversation to defend them. It's total rot.