JFBConnect can let a user authenticate through Facebook (and the other social networks). If you have the "Enable Automatic Registration" setting enabled in JFBConnect, then a user's account will automatically be created. If that's disabled, they will go through a registration flow where they need to fill out any missing information.
If the user isn't clicking the "Facebook" button to login/register, then JFBConnect has no part in other registration flows. If they click "Create an Account" in JomSocial, and they aren't able to register that way, the issue likely isn't related to JFBConnect at all. In that case, you'd need to contact JomSocial to determine the problem.
I hope that helps explain more,
Alex