I was using:
.fb-comments, .fb-comments span, .fb-comments iframe[style] {
    width: 100% !important;
}
To make Facebook Comments responsi
Ok this is what I have so far based one Timothy's comment.
function resizeFbComment(){
  if (typeof FB === 'undefined')
    return;
  $(".fb-comments").attr("data-width", $(".fb-comments").width());
  FB.XFBML.parse($(".comments")[0]);
}
$(window)
  .load(resizeFbComment)
  .resize(resizeFbComment);
Obviously, this is a temporary hack. The windows resize should have a timeout.