I use Facebook, MySpace, and LinkedIn.
Facebook primarily keeps me in touch with high school and college friends.
MySpace isn't as focused on school friends, it encompasses more people.
LinkedIn - This site focuses on professional connections. I've had it for a few years but haven't used it much because I don't know many people on it. Leo Dion just sent me this article on LinkedIn and I've decided to ramp up use of it. I think you should read the article and then decide if LinkedIn makes sense for you.
I've heard there are websites which will aggregate social networking sites (similar to the way Trillian works). I haven't found any of these to try out.
LinkedIn supports importing contacts from email clients which may help you start your network. I use Gmail. I had to export my contacts to csv files and then import them into LinkedIn to get it to work.