Like SC said, I use those sites to keep in touch with friends I lost touch with--friends from high school and etc that I may not regularly talk to any more. Also, I use it to keep in touch with some of my relatives--a few cousins have MySpace sites and so it comes in handy to be able to see what's going on their lives on a day-to-day basis and know they're okay.
Also, Facebook has a GREAT feature I use on a regular basis--the event planner. I use it to invite people to basically every event I do/throw/etc. It lets me keep a running tally on who's coming and who's not, and it lets me let them know in one, referrable place what I need from them
Sure, I could call up and invite people over the phone, but that's 20 phone calls I don't have time for. With Facebook, I can send out ONE invite and save a ton of time.
Personally, I only friend people I know--either people from real life or people from TFP. That's it. I'm not into randomly collecting friends I don't know from around my unversity the way some of my friends have. That's just not my style.