This is a CSS only solution I found online. Add the following code to your .css file:
.fb-comments, .fb-comments iframe[style], .fb-like-box, .fb-like-box iframe[style] {width: 100% !important;display:block;z-index:2000;position:relative}
.fb-comments span, .fb-comments iframe span[style], .fb-like-box span, .fb-like-box iframe span[style] {width: 100% !important;display:block;z-index:2000;position:relative}
When you get the Like Box code, you need to remove the width in the source link attributes and change it to 100% in the iframe properties:
<iframe src="//www.facebook.com/plugins/likebox.php?href=https.............&
width=200&height=395&colorscheme=light&show_faces=false&header=false&stream=true&show_border=false" scrolling="no" frameborder="0" style="border:none; overflow:hidden; width:100%; height:395px;" allowTransparency="true"></iframe>
It works!