On the developer page (http://developers.facebook.com/docs/reference/plugins/comments/) I can see that the facebook comment Social plugin has the following attributes.
Attributes href - the URL for this Comments plugin. News feed stories on Facebook will link to this URL. width - the width of the plugin in pixels. Minimum recommended width: 400px. colorscheme - the color scheme for the plugin. Options: 'light', 'dark' num_posts - the number of comments to show by default. Default: 10. Minimum: 1 mobile - whether to show the mobile-optimized version. Default: auto-detect.
For mobile it gives me the default value, but what are the others? False/True, Yes/No, On/Off, None/ ????
You can turn this behavior off by setting the mobile parameter to false. see the mobile version ignores the width parameter, and instead has a fluid width of 100% in order to resize well in portrait/landscape switching situations. You may need to adjust your CSS for your mobile site to take advantage of this behavior. If preferred, you can still control the width via a container element.