Then in the future -- every time you return to MVS -- you'll be logged in to your MVS account without having to log in. As long as you're logged into your Facebook account. (Not that logging in to MVS is a terrible burden anyway, but hey.)
The idea is that Facebook is more pervasive than MVS, and thus Facebook authentication is more pervasive than MVS authentication. It's like a passport that whisks you into MVS.
If this is confusing, forget you ever read it. You're not missing anything.

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