I have just discovered Facebook and frittered away most of my Sunday mucking around with it *shaking my head* :D
I highly recommend joining up despite the time suckerage. I've never really participated in social networking sites, but this one had me hooked with its ease of use and entertaining functions. I especially like the Tetris competition against friends.
I could rattle on a bit but, best if you just jump on and give it a go. I toyed with Friendster upon an invite a few years ago but left it alone soon after. I haven't investigated other social networking sites but from all accounts, Facebook is a player that is changing the game.
There are a good grab of screenshots on this page if you want to have a look-see before joining.