It sounds like there's some issue with a stale row in the EasySocial database table. When you're testing, are you registering, deleting and then trying again? I'm not sure why that row would already exist if this is the first time the user is registering on the site.
My other suggestions beyond the above is:
* Test with the "Automatic Registration" setting of JFBConnect just to see if that properly creates the user
* Try re-ordering the JFBCSystem plugin to the top and bottom of the system group, which may re-order parts of the registration flow better
* Ensure that the Facebook authentication features of EasySocial are disabled
I hope that helps get you started, but if not, please let us know what versions of Joomla, EasySocial and JFBConnect you're using and we'll help investigate further.
Thanks,
Alex