I have accounts on several sites, I never even go on linkedin though and have never really built much of a profile. I have had a myspace account for years. I got invited by a cousin who went to school with Tom, the founder, when it first started. During my research for schools I looked up profiles of current students and alumni from schools I was interested in. Amazingly most of the profiles I would describe as very unprofessional and pretty much stereotypical frat boyish in their content.
I do have a friend who works in HR for a major company and they do check people out on myspace since it can be much more telling than an interview. So with that knowledge I wouldn't put anything on there you wouldn't show your boss or your mom without being embarrassed.
I keep mine since its a great way to get back in touch with people and stay in touch. I have reconnected with people I went to high school with and haven't talked to in a decade. One of my friends actually met back up with his first childhood girlfriend and they are married now. So it has its good points but then again I am not going to post pictures of myself doing something compromising to be funny and I would write anything about myself that I wouldnt tell other people. I dont add people I dont know or that I wouldnt want to know. I delete comments that are in poor taste. Its what you make of it, in reality a Linkedin account could be a usefull tool in school to maintain a network and connect with alumni.