I'm not looking for a date, so you won't catch me on friendster but the 6-degrees-of-separation thing is nothing if not entertaining.
These are some of the social network entities I've joined... The value derived from joining these things has yet to prove itself but it's kinduva an interesting idea. So far, I've introduced one friend to a recruitment contact via another friend at a company he was interested in. I hooked up contact with another friend from long ago who I'd lost contact with. So, these things aren't useless but nothing's hit it out of the park yet.
The social networks consist of the people you know through various social haunts and shared activities or you worked with in your day-to-day toils.

These are some of the virtual communities and secret societies I'm involved with too: