Manipulating the Facebook Comment Social Plugin to appear full width is a bit tricky.

It requires to add code to your stylesheet

.fb-comments, .fb-comments iframe[style], .fb-like-box, .fb-like-box iframe[style] {width: 100% !important;}
.fb-comments span, .fb-comments iframe span[style], .fb-like-box span, .fb-like-box iframe span[style] {width: 100% !important;}
.fb_ltr {width: 100% !important;}