Sixdegrees.com, one of the first social networking sites, back in college. I have been using them long enough that just talking about the time-suck that is Facebook will not cut it. Instead, I'd like to talk about a few the more obscure social networks I use, and what's neat about them.
43Things is a niche site in some ways, organized primarily around the question: What do you want to do with your life? It's sort of a barebones blogging platform centered on personal goals both large and small. Each goal you add to your list is a link; by clicking on this link, you can see a list of all the people who share that goal and read about their progress on it. You can write as many updates as you want on your own goals, cross them off your list, or revisit them at a future time. People can easily adopt other people's goals, comment on other people's entries, and give out a limited numbers of "cheers" daily to encourage fellow users.
It's turned out to be a very personable, mostly very positive sort of site, and I find that it can lend itself to quite meaningful relationships. A few of these have even turned into IRL friendships for me. Even if you don't use it to build relationships, reading about someone else's methods and progress toward a mutual goal can be invaluable. It can be as serious or as frivolous as you decide.
Last.fm is really about the music, first and foremost, and there are a lot of places on the Internet where you can stream music for free. The social networking features of Last.fm are what make it my music streaming application of choice. As I listen to music on my own computer and rate music that is broadcast via the free music stream (quite customizable), the site automatically builds a profile and library of my musical taste. As it 'gets to know me better', it matches me up with other users whose tastes are similar to mine.
One of my favorite things to do is to listen to the "My Neighbourhood" station - it plays tracks from the libraries of people who share my taste but have much much better musical collections than I do. :) As a result, I like almost everything I hear on the station, but most of it is 100% new to me. I should probably take a break from it for awhile, as the mp3 portion of my entertainment budget has increased substantially since I started using it. There's just too much good music out there.