> AFK you can change your nickname in twitter; if you do that then you will not be able to log into funky world cup. That is why I would store and authenticate using uid and store nickname in the :name property.

Good catch. I like adding a nickname field approach. The only thing I can think of, about using the user ID, is if there are any chances of Twitter's user ids and Facebook's user ids to conflict with each other.
If this possibility exists, we need to use both, provider and user id to authenticate.
Do you know about this for sure?