I just tried to register as well. I never saw the JomSocial registration page. Instead, I was brought back to your home page with the following system message that appeared briefly and then vanished:
Facebook profile successfully connected.
You are not authorised to view this resource.
I am logged in at this point, so things seem correct.
If I go to
www.atjump.com/en/my-jump I'm immediately redirected back to the home page.
To me, it seems like there's a permissions issue with the my-jump menu item (or whatever you've set as your redirection URL) or there is something else (like the JomSocial redirect plugin) which is trying to force the user somewhere else. Try disabling that plugin altogether to see what happens.
As for the JomSocial registration flow, I'm not seeing that. If you are, it probably means that you're logging in with a social network account using an email address that already exists for a Joomla user on your site *and* you don't have the "Automatically link users by email address" setting enabled. In that case, the only thing JFBConnect can do is let the user register manually and add a different email address.
I hope that helps,
Alex