In the ACP Member management area, there should be an indication somewhere showing that a member has added a facebook/twitter/whatever account to their forum account. This will make it incredibly easier to troubleshoot external issues (such as members accidentally creating duplicate accounts when they use the facebook interface) as well as to identify who is actually using it and be able to know what is what. The biggest part of this though is being able to see that yes, User A has already connected their account to a facebook account, before I attempt to merge (or delete) what appears to be an accidental duplicate account.

The member list in the admin CP shows what you are asking for. There is an image next to the name that shows twitter & facebook. Take a look at the members tab, and members list.

Yes, this was added in 3.1.2 (though I *believe* it's only configured to show Twitter/Facebook, and not any other methods).