JFBConnect already includes that CSS to make the comments responsive. However, Facebook made a change to their Comments widget in early/mid March that broke all ability to override their CSS and fixed the width of the widget. You can see their bug report, below, which they've accepted. We're hoping it will be fixed soon:
developers.facebook.com/bugs/256568534516879/
If, when fixed, there are updates required in JFBConnect for responsive comments, we'll definitely implement them.
Thanks,
Alex