Filipe,
I'm not sure. That's not a common issue.
I just tried to register on the site. Can you tell me if my account was created and is approved in both the Joomla User manager and in Community Builder? I'm not able to login in now either, though if I refresh enough times, it does work.
Does your template display system messages (those text messages in a blue or red bar)? If not, or you've never seen them, try switching your template to one of the standard Joomla ones and logging in that way. JFBConnect should display the reason you can't login when it won't log you in. Additionally, in JFBConnect, please try enabling the "Display Facebook API Errors on Front-End" in the Facebook API tab of the configuration area.
Hopefully, something above will help us narrow down the problem.
Thanks,
Alex