I've joined Facebook, Myspace, LinkedIn, Friendster, Orkut, Digg, Flickr, Twitter and a bunch of others. The verdict... Social network sites are pretty useless for socializing.
Don't get me wrong these sites are entertaining, voyeuristic and in many cases addicting. The primary function on most of these sites is as the name implies 'networking.' I get invites from people to add them as a friend, buddy, pal, co-worker to their list of friends, buddies, co-workers and pals. The social graph grows but little changes.
Do you have more real world friends because of these social networks? Probably not.
What these sites do is allow me to spy on people. I can see what they like, what they do, who they know and what photos they take. No more secrets.
Many of these sites don't focus on the 'social' part of their name. Evite is the best 'social' tool and I'm not even sure that they consider themselves a social-network. I would say I've met more people because of Evite then from all the other social networks combined. Evite works because I didn't have to accept networking relationships. Instead I show up. The experience is focused on the fun part of socializing, not the boring part 'networking.'