All of the mentioned ones are cool (I use facebook), but like others have said, they are geared towards finding and socializing with already established friends - or people you meet through your friends.
A few cool sites that seem more like what you are talking about, try these:
Tribe 43 Things Instructables
The last one is more of a DIY'ers social networking site - but, I find it alot of fun to just participate in the discussions of certain projects, is definitely a very social site.
Tribe will probably be your best bet, I like the concept alot and the people are fun.
43 Things is also a cool concept and uses social networking around a '43 things I want to do' list concept (should appeal to Pavlina goers).