The real reason social media sites die without moderation is not because it alienates people who take bullying, it is that when there is no way to become a moderator, people seek status and power elsewhere. Reddit survives because it is a status pyramid scheme. HN survives because of similar aspiration to be seen as that smart person in front of a VC audience, a kind of Enders Game fantasy.
Facebook and Twitter are waning not because of trolls, but because they have signaled a nannying supervisory role that reduces overall user optionality, and takes away the thing that drew people in in the first place: Hope.