Ivo,
Just got in to really poke around and see what's going on. Unfortunately, I'm a bit clueless. JFBConnect tell Joomla to create the user and really just sets up the parameters for that user's creation. We debugged our code to make sure everything is happening properly on our end, and it is.. up until we call $user->save(), which should tell Joomla to do it's thing. As you (correctly) diagnosed, the row in the jos_users table is being created, but the ACL rows aren't, which is why the user isn't showing up in the User manager or basically working in general.
We've never encountered this issue before, and aren't really sure what's going on. We know you have AdminTools installed, which shouldn't be the problem, but may be worth disabling it (I didn't want to), since it doesn't have some security stuff in there.. but nothing that I know of that should cause this.
In general, if you know of any specific extensions or configuration settings that may alter the user behavior, please let us know. I tried to diagnose so many common issues, but didn't come up with anything. The other option is to try on a different Joomla site, if possible, to see if the issue is there. Something's definitely odd, but I really don't have much a clue.. sorry to say.
Finally, note that you may need to keep deleting the user in the jos_user table before trying to create again, or else JFBConnect will see that user and try to simply connect the Facebook user to that account based on the matching email address... don't want that to cause problems.
Keep us posted on anything you find or any ideas you have. We'll look through our code a bit to see if there's anything we can do better, but not sure what we'll come up with.
Thanks,
Alex