I'm 24 and not yet fully immersed in the professional world. My profile is private, but I'm not terribly strict with my screening process for friend requests. If I know the person and have gotten along with them in some context, I'll usually add them. I never go add someone I only minimally know.
I keep my profile pretty sanitized. I have more family on there as friends now, so my status updates, though not bland, usually omit profanity and stuff like that. I never got into the "post 320 pictures of me boozing" thing, so I'm good on that front.
People should be aware of what content they post and allow others to post concerning them. I've met a couple of people who thrive on making examples of people for letting anyone with the 'net see anything incriminating about themselves.