As I see it, Facebook and Google actually do one better than a social contract. They have a real contract with every individual user in the form of the EULA. I think you could even say they have a social contract too, that being the public's general understanding of what Facebook/Google offer and what they give in return. The argument for societal consent to Google/Facebook seems very easy compared to societal consent to government authority, even in a democracy. But I don't put much stock in social contracts as an ethical argument anyway. I tend to agree with Lysander Spooner's arguments, e.g. on the Constitution: "No one's consent could be presumed against him, without his actual consent being given, any more than in the case of any other contract to pay money, or render service."