How would they know?
If you call yourself "The Immortal Khan" - okay, it's a giveaway. If you call yourself John Smith, however? Or, as I did for one account, my middle name and the translation of my surname? Or, given that under Scots law it's legal to take any name that you regularly use as your legal name, with no form filling, a new one every six months?
In the end though, Zuckerberg and his lawyers can, and probably will, say that Facebook is voluntary, and if you don't like the rules, get out of hte social medium. And he does have a point.