Suppose Facebook or Twitter wanted to be really good netizens, and let you use their service to log onto other services, but to not lock you in. How might that work?
Well, I wouldn't be known on other nets as @davewiner, instead I would be known as dave.scripting.com. That name would be a CNAME for Facebook, if I was using Facebook to guarantee I really am who I say I am. But if, in five or ten years, I decided to use another service, one with a neat feature Facebook doesn't offer, I could point dave.scripting.com at the other server.
It would be like changing your credit card number, something we have to deal with from time to time. Not a great thing, but then not too bad either. You get reminded of all the things you're paying for automatically. I actually like to go through that ritual from time to time. In this case you'd be reminded of all the services that have access to your personal stuff.
I doubt if Facebook would do this, but then a commercial ID hosting service, one that charged me say $10 a year to be my identity server, might be willing to offer such a service and make it easy for me to switch.