A thought to get down here: Twitter is in many ways a community, the way a BBS is a community. But the loosely coupled nature of the community means that Twitter has no way to protect itself from the effects of scale. If there is a summary for the Tragedy of Twitter, this is it.
Facebook is different than Twitter, both because it is less of a community in itself: it is more a home for different communities, There’s not really a Facebook identity the way there is a Twitter or Tumblr identity.
For a story of a facebook group implosion, see Death of the Longest Shortest Time Mamas
Here are some suggestions on Reducing Abuse on Twitter, which involve giving the community some tools, albeit on an individual basis.
Researchers have found that Anger Spreads Fastest through Twitter-like networks.