The problem is that offline, there is an implicit vetting system. I.e. if in your group of friends, one of the guy is always trying to hook up with a different

The problem is that offline, there is an implicit vetting system. I.e. if in your group of friends, one of the guy is always trying to hook up with a different girl everytime, quickly people will stop inviting him to events. However, some social networks that have become "implicit dating websites" (meetup.com comes to my mind to some extent), there is no such self-regulatory system.