I am building a website for a private community, but would like to integrate facebook and eventually Google+ in order to facilitate the "open" communication of the members via social networks. Now, all members must identify to the service. For users choosing to register via the normal route, we rely on the fact that they all have access to a certain vanity email address that we can check before we approve them.
Obviously, in the case of facebook, we can't do that unless they so happen to use that email as their primary facebook email.
Since this isn't the only circumstance in which you would like people to tell you something at the point of registration/login, I'm bringing this up as a suggestions instead of simply hacking something together for my own use. Obviously, we could go through the trouble of contacting them after they have registered, but adding fields would make things easier -- and could be useful in all kinds of other situations, too. If only to fill certain "special" fields that your community uses and that facebook couldn't fill.