The following CSS will allow for a fluid width for Facebook Comments.
//Edit
Must add a new data-width attribute.
.fb-comments
{
width: 100%;
}
.fb-comments > span
{
width: 100% !important;
}
.fb-comments > span > iframe
{
width: 100% !important;
}
----html
<div class="fb-comments" data-width="100%"></div>