Same issue here. Both the WP and G+ comment areas are responsive when you manually adjust the width of your browser window.
The FB comment block STAYS the width of the browser window when the page was loaded.
So.... technically, this really won't be an issue for users in browsers or on mobile devices because they width of the FB comment block will display at the width of the parent bounding element when the page was loaded. This is ONLY a problem if you were to rotate your mobile device from landscape to portrait orientation, or if you manually resize the browser window on your computer.
Just FYI, by default, using the FB Comment plugin from FB, if you do NOT provide a width value in the settings, it will expand to full width of the parent element AND WILL be fully responsive when you manually adjust browser width.