> The thing is to have a common Trusted Third Party, and somehow keep the number of such parties large enough.

Aye, there's the rub. What's to stop most sites from only allowing Google/Facebook as the Trusted Third Party? And you also need to worry about security breaches, or one company quietly buying up all the independents, or governments legislating in back doors, or every service you use ganging up behind the scenes to try and collate your "anonymous" public keys back into a single identity.

Don't get me wrong, I do think there's a way forward, but it's not going to be easy.