I opened an account in my real name in order to view an online contact's photo gallery, which she'd invited me to do. Within about 20 minutes I got suspended for 'suspicious activity' and, despite having sent proof of my ID to Facebook several times, my account has never been reactivated. I'd never even posted anything!
Facebook's rules prohibit the use of fake identities and they close around 20 million accounts per day because they suspect that those users haven't used their real IDs, but plenty of people still seem to be able to join under an alias anyway.