You won't like this solution.. But I think the most effective way to fix the site would be to delete everything and start from scratch as a referral-based invite-only site.

There is a reason communities fork themselves as they approach Dunbar's number. It's not that we lack the technology to talk to more than 300 people at a time, it's that we implicitly treat strangers differently than we do friends. The problem is sociology, not technology.