Bacon,
That's covered many times in this thread already (I asked the same question many pages back) ... the way it works is this - you can choose a new nickname, or if you already are registered, you can choose "use existing nickname or ID".
So, for members who are already registered, but wish to connect to their FaceBook account, they should enter their FaceBook credentials, then on the next screen, choose "use existing ID".
Great app - FaceBook.Com is now consistently the #1 referring site in our Google Analytics for
http://www.theworshipcommunity.com