A few suggestions (which would probably need to be limited to over X age): * Google accounts * Github accounts * Twitter accounts * bitcoin-otc accounts (pr

A few suggestions (which would probably need to be limited to over X age): * Google accounts * Github accounts * Twitter accounts * bitcoin-otc accounts (probably with a minimum reputation) * GPG keys with some minimum threshold of age and "signedness" * S/MIME certificates from issuers that verify government ID. I'm guessing the difficulty with supporting multiple methods is a desire to limit this to 5k/person. I'm not sure there's a good solution to this, though I will say that you may have just added some additional incentive to steal Facebook credentials.