These are exactly my thoughts as well, both the positives (it doesn't need to be air-tight) and the negatives (providing documentation). I don't know that there is a great system here. The best I can think of is having independent third parties that people can register with and that can provide a 'proof of eligibility' token tied to an e-mail address or something similar with the explicit, backed by law, understanding that sharing more than that proof of eligibility with a third party is a criminal offense. The money side of things would be that FB and the like would pay the proof company a service fee so they make money and FB gets the proof without getting access to your documents. Just a thought.