First of all you can be sure that this is because some kind of a very clear reason.
Second - you don't know that Facebook and Big Brother were not working together from the start. Just because Zuckerberg was against giving access to their data (publicly), does not mean that it's actually true.
Last of all, Facebook is a socializing website, what's the problem if people are using fake names? They're called nicknames and were used since the beginning of the internet. Is Zuckerberg on some kind of power trip? Or he's out of the whole deal and people there are doing whatever they want?