In my opinion, this is as strong AI problem, and only people have strong AI. Therefore, large social sites are always going to need human moderators to have a good signal to noise ratio.
That's a bit generous, I think. Most people aren't smart at all. And even the ones that are don't act objectively and without bias. Ever. Although I will admit that moderated solutions almost always come out better. See slashdot's complete lack of this problem.
Very correct. In fact, nearly half of all the people you'll meet have a "below average" intelligence.