Of course you can. Contracts can and do override a right enshrined in law. I can sign an agreement that gags me from talking about something, in which case I've given up my right to free speech (if you're in a country where such a right exists).
You seem to be referring to the EU's Data Protection and privacy policies - there are explicit exemptions under the EU/US Safe Habor (which applies here) that allow contractual opt-out from certain rights. I am not saying this is the case with Facebook: merely that it is naive to assume just because a right is enshrined in law doesn't mean you cannot give it up through a contract.