With the huge focus on Responsive design, it is really important to make everything responsive.

The Facebook Comment Box API isn’t responsive ATM.

.fb_iframe_widget,
.fb_iframe_widget span,
.fb_iframe_widget span iframe[style] {
  min-width: 100% !important;
  width: 100% !important;
}

Code taken from

http://stackoverflow.com/questions/22187961/responsive-facebook-comments-css-hack-broken