I guess the biggest problem is that Facebook is now the place where everything comes together. You suddenly end up having your boss, parents, coworkers as friend along with the guy you used to do drugs with during your teen years and that trashy ex-girlfriend. And it's not easy to keep those things apart with the real name policy facebook has. It's also not easy to reject a lot of friend requests. Sure somebody you haven't seen in ages you can easily ignore but with coworkers it can get unpleasant. I'm glad that my parents are too old for facebook but for a lot of especially young people rejecting a parents' friend request is probably impossible as well.

Sure you can try to have different profiles. But they easily spill over. And facebook collects a lot of data. The worst thing are probably the phone and address books of your friends. Someone has stored your email address or phone number associated with your real name on his phone and installs the facebook app and suddenly they now. Or people are stupid enough to give facebook access to their email account. They even started to ask friends of you what your real name is.

I recently had to delete my account because they realised I was using a fake name. I don't know if somebody "squealed on me" or they got the information elsewhere.

In the past your online identities were easily separated because everything was in a different place and it was easier to be anonymous. You could have several IM profiles for friends and family separated. With other people you'd only communicate through one of your many email addresses.