I wholeheartedly believe that more social sites should use 'invite trees' as their primary abuse prevention mechanism.

It doesn't even need to limit the amount of invites, or anything like that - all it needs to do is keep track of who brought in a given user. It's a minimal invasion of privacy, and allows you to eliminate large swathes of sockpuppets and spam accounts at once through their common ancestor.

This is something that startups are unlikely to implement, because it would conflict with their "userbase growth at all costs" goal. But community sites have no such constraint.