There are a few different ways to address this:
1. In JFBConnect backend > Configuration > General tab > Normal Registration area, you can set the options for JFBConnect to generate some of the fields for the Joomla user (such as username, password, etc). These can be hidden from the view to simplify the registration area.
2. If you're importing social profile information into JomSocial and have the 'Registration Component' option set to "JFBConnect", you can update what fields are shown through JFBConnect. Go to JFBConnect backend > Profiles > JomSocial tab. Then set the 'Show Profile Fields' and 'Show Imported Fields' appropriately.
3. If you have the Registration Component fields set to JomSocial (or depending on how you've set the options in #2), you can also limit which JomSocial fields are shown by editing the fields in JomSocial. Each field has a 'Registration' and 'Required' settings that will affect what's shown.
I'm sure that's clear as mud, since my coffee hasn't kicked in yet
So please let us know if you have any further questions.
-Melissa