I just tested the site. After the iFrame asking for my Facebook credentials, I was successfully redirected to:
http://www.domain.com/investors/index.php?option=com_jfbconnect&view=loginregister
I then filled out the registration side of the form and created a new account. I was logged in successfully. I logged out, and then was able to log back in with the same Facebook credentials.
I'm not sure how you're locking down the site, but please test with it 'open' for now and see if that helps. As far as I can see, everything is working exactly as it should.
We used Chrome on a Win7 system. If you're testing and seeing differently with another configuration, let us know what it is, and we'll test with that.
Thanks!