Some people have both an Instructables account and a Facebook account. Some of these use different email addresses for each. When a person clicks the Login with Facebook button and approves Instructables with their Facebook account, one of the pieces of information we get is their email address, and we check it against our existing email addresses so as to link the correct account if we find a match. Often there is no match, and in the vast majority of these cases the person doesn't already have an Instructables account. So we make them one, and link it to their Facebook account.
However sometimes they do have an Instructables account already, and now all of a sudden they have two! This is confusing and wrong, and we'd been dealing with it in a rather arduous manner involving direct database changes, which is always something to avoid. Now we've got a system for admins to handle this case with just a few clicks.
Other than that, there are some bugfixes, for example around character display -- a few of our new Facebook visitors don't use the Latin alphabet! So we handle that a little better now. We have some more sensible default actions for new registrations, and a couple things to help our admins manage contests more easily, which they've been asking for for some time.