Hey, the exact same thing happened to me. I have the earlier (pre 3.8) version installed on a 3.7 forum (
http://www.theworshipcommunity.com) and it works perfectly - including the ability to use your existing iD ... everything works great.
I just installed this plugin again (the 3.8 version on a new 3.8 forum at
http://www.e-businessonline.com) and I'm seeing the same issue ... I choose "use my existing username", I then enter my vbulletin userID/password, and it logs me in as the normal user - does NOT update my FaceBook page.