I was using:
.fb-comments, .fb-comments span, .fb-comments iframe[style] { width: 100% !important; }
To make Facebook Comments responsi
try using this code This might be a little different
#fbcomments, .fb_iframe_widget, .fb_iframe_widget[style], .fb_iframe_widget iframe[style], .fb_iframe_widget span, #fbcomments iframe [style] { width: 100% !important; }