this kinda thing has been asked before, you might find other answers if you look it up. i have a facebook, but i don't use it that often. i don't really like the apps. i have a bebo (which has also, annoyingly, introduced apps) and i much prefer it over myspace.
and to actually *answer* your question ;) ...
i use it to keep in touch with others i know on the site, and for others to keep a tag on me - it's nice to know things about people without having to ask them all the time.