Is the new Facebook login to DNN only meant for registering for new accounts, or can you use it to login to an existing DNN account with the credentials of an existing Facebook account?

I ask this because the later would seem to be the preferred approach, but if so, what needs s to match up between the two?

I have tested this with my own Facebook account, and the first time it created a new account on my DNN site with a different username. I then created a DNN account with the username that Facebook was attempting to use, and it connected! Tried again with my wife's account, and it did not work...

The current documentation is very vague on this topic...