A "group is its own worst enemy". -- Clay Shirky. Justly famous post by Clay Shirky on the way groups work to defeat themselves. Some ideas from it are itemized below. post 
We want to see the user of software as the individual, as we do with desktop apps, but, as Shirky notes, in social software The User Is the Group.
Cohen's Law predicts unmoderated groups will spend increasing bandwidth arguing about moderation.
Soft-forking Groups like those on Twitter, LiveJournal, or Tumblr have blurry edges and respond well to scale.