One of Facebook’s most excellent “secret” features is that it is actually doing OpenID while everyone else is merely talking about it
I have joined every Facebook App possible (check for yourself) and I have never needed to register for any external 3rd party app. Facebook is acting as my proxy and is signing up on these external application on my behalf; additionally, Facebook integrates the external application and takes care of all authentication based on my having logged into Facebook just once.
In other words, Facebook is actually becoming a highly-trusted change-agent for OpenID trust nets while all the ubergeeks are mentally-masturbating about it.
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Using http://identitu.de you can extend this further by having an OpenID proxy over your Facebook account :) So you can use your Facebook account at 3rd party sites supporting OpenID but not running on the Facebook platform.