i'm on friendster, myspace, and orkut. don't like orkut.
friendster is great, i've foiund a lot of old friends from as far back as elementary school. very professional looking, good site. and they've been adding content like chat forums (which i thought weren't implemented great, but at least they'r ethere), and instant messaging.
myspace is like friendster, but for, in general, a more alternative crowd (in my point of view). it's a great site as far as features go. if friendster could add all of the features and functionality of myspace, i think ti would be super duper!
myspace is really more towards 'friend whoring' (getting as many 'friends' on your list as possible) than anything else i think.
but their all free... id' definatly recommend friendster and myspace, but if you really wnat to keep in touch with friends, use friendster.