When embedding facebook comment plugin into your website. You will find out the iframe API supported by facebook like that :
<div id="fb-root">
<script>(function(d, s, id) {
  var js, fjs = d.getElementsByTagName(s)[0];
  if (d.getElementById(id)) return;
  js = d.createElement(s); js.id = id;
  js.src = "//connect.facebook.net/en_US/all.js#xfbml=1&appId=216059938523158";
  fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));</script>
<div class="fb-comments" data-href="http://example.com" data-num-posts="2" data-width="470">

 You can see that, the width and height attribute of fb-comments div are declared in pixcel, so when you make your website's interface in responsive way, it will break out the layout.

You don't know exactly the parent of fb-comment div width, in fact it alway change when you see in different screen.

How can we solve out this problem ?

Just try this css script :

 .fb-comments, .fb-comments span, .fb-comments.fb_iframe_widget span iframe {
    width: 100% !important;

Want to know why ?  Plz comment at this entry ^^