Pluggin: Facebook Comments for WordPress (actuuly this one is the TOP RATED one. With 250,000+ downloads, so you shoulkd consider to useTHIS ONE with wordpress).
THE ISSUE (which i, and most ppl) had: After the installation, the plugin only showed "LIKE" button an the end of the cage, but no "comments" section appears.
THE SOLUTION: Install plugin - go throght "normal" settup proccess, (where you should provide the pluggin with your ID and app secret ID).
Yeah yeah, i know, this is by far the stupidest glitch ever. Plus, the box itself have a comment "(This should be checked unless you've manually included the SDK elsewhere)"
Why doesn't developers "enable" that check-box by default if they tell so by them-selfs - i got no F idea. Never-mind.
Worls for me well. At least it words with "SHOUTBOX"
Check \ test page: http://imba-shopping.com/3-post-test/