I signed up for facebook back it when it was brand new, I think I have about 75 people on it now. The vast majority of those are people from my old high school, maybe 50% of that group are people I was more than a faint acquaintance with.
The way it's been re-designed, there's not really any point to looking at people's profile pages. I just log in when I'm bored and see if people posted any new status updates or photos.
It's trendy to message people and add applications and make tons of wall posts, but you're really not expected to do anything at all, you can just observe. I know "popular" people who might have lots of friends, but very stripped-down facebook profiles and they hardly log in just because they don't feel comfortable putting much online.
Now Myspace is something I don't understand. As far as I can tell, its only purpose is for teenagers (because that's where are all the other teens are), to put up a quickie web page for your band (don't have one), or to pick up random girls for casual sex (don't know how to do that). Actually, that's how I first heard of Myspace, my friend wanted me to get an account and log in so I could "rate" different girls he was talking to. That was what, 4 years ago now? I feel old.