Set the Width parameter to '100%' and it should be responsive. Facebook goes back and forth on making, and breaking, the responsive behavior of the comment box, but that is the recommended way to make it responsive properly.
I hope that helps, but if you need anything else, just let us know.
I'm glad to hear we could help get you going. If you need anything else, just let us know.
Finally, if you haven't already, please consider leaving a rating and review for JFBConnect, or our support, on the Joomla Extension Directory. It's certainly not required, but very appreciated:
http://extensions.joomla.org/extensions … jfbconnect