Kevin,
The mobile=true doesn't change the widths, just the looks and a little of the functionality of the comments. Unfortunately, the Facebook widgets aren't perfect in a responsive template.
With that said, if you add the following to your CSS file, it should help a lot. It's something we're planning on adding in a future release as well:
.fb-comments, .fb-comments iframe[style] {width: 100% !important;}
Hope that helps. Please let us know how it goes, we'd love the feedback.
Thanks,
Alex