Possible to have Facebook Only?

I *believe* you can simply disable registration in the Global Configuration area of Joomla, and that will do what you want. We actually disregard that setting (intentionally). Joomla (of course) and (I think) JomSocial respect that setting, but I could be wrong about JomSocial. In case they don't, you can simply setup a redirect within Joomla's Redirect component from their registration page to another page on your site (possibly one that is a simple Joomla article saying "Register here" with a {JFBCLogin} tag..?)

Hope that helps get you started, but if you run into issues with the above or have questions, just let us know!

Thanks,
Alex