Yes, but not everyone is at the same level when considering what's appropriate and what isn't. Just because someone's an "adult" doesn't mean they're obligated to behave "adult-like". There's a difference.

There's only so much you can do. Is your Exec Board allowed to slap them with fines and/or put them on restriction if they don't comply with your chapter's internet policy? Personally, I don't believe in either of those examples and I really think it might make the situation worse.

Maybe I'm saying this because FaceBook was just in its Beta stages when I finished college, but if you're concerned that PNMs might look at you folks negatively because of a few FaceBook profiles, then you're probably be better off without them anyway. Focus your time and effort on putting on the best recruitment ever and don't waste it worrying about a few bad profiles.