There are a lot of different social networks out there now on just about every topic. In my browser right now I even have folder dedicated to the ones I do my best to follow. Here's my list:
(not really in any order)MySpace - there only 'cause a few friends are
(promise!)LinkedIn - best business networking out there, love it
Spock - recently invited to, hasn't tickled my fancy yet though...
MyBlogLog - used to be fun, now mostly filled with people putting up ads for themselves.
Facebook - have both friends and online contacts here, pretty interesting
Now you might say, "Why join so many?" well with some, they serve a specific purpose. With others, it is good to get your name and "brand" out there in front of the public 'cause it might bring in a few new people to your web sites.
What about you? What social networks do you enjoy or get something out of?
I personally prefer: Facebook, StumbleUpon and Twitter and last but not the least Digg somehow. But as per personal views and personal experience: Facebook and StumbleUpon sre the best sites for socials. It will give you business and traffic respectively. So I personally go for these 2 websites.