This is why to solve the Facebook / social media silo problem, we really need to solve the web identity problem.
The solution needs to allow identities which are "register once, use anywhere" across the web, and portable so that you can migrate to a different identity host/provider/implementation without losing all your accounts. Ideally these identities would also allow you to reveal as much or as little information about yourself as you want, and not force you to reveal some unique property which is correlatable between colluding sites.
Obviously that's not an easy thing to create, and may actually be harder than creating a viable rival to Facebook, but if we don't solve the web ID problem, then governments and corporations are going to "solve" it force us, and we'll all be the worse as a result.
I liked Mozilla's Persona (BrowserID) for the fact that it protected privacy from the 3rd party provider. You're asking for privacy protections from the site seeking identity authentication.
It's a fascinating idea to have some means of identity authentication where the party seeking to authenticate your identity doesn't have enough identity information to connect the dots, and the party providing authentication doesn't even know about the party seeking authentication.
The solution needs to allow identities which are "register once, use anywhere" across the web, and portable so that you can migrate to a different identity host/provider/implementation without losing all your accounts. Ideally these identities would also allow you to reveal as much or as little information about yourself as you want, and not force you to reveal some unique property which is correlatable between colluding sites.
Obviously that's not an easy thing to create, and may actually be harder than creating a viable rival to Facebook, but if we don't solve the web ID problem, then governments and corporations are going to "solve" it force us, and we'll all be the worse as a result.