Q.Mark, I strongly recommend that site owners not use the Facebook Social Plugin to support commenting on their sites, unless they want to annoy visitors.
Facebook uses an Orwellian blacklist/censorship model which prevents many people from posting comments. Users get no explanation for why their comments won't post. Rather, it's made to look like a malfunction of the web site, and it is only apparent after the users waste their time composing a comment.
There's discussion of the problem here (among many other places):
https://www.facebook.com/help/community/question/?id=10200248881692914
(Copy saved here:
http://burtonsys.com/Why_cant_I_post_comments_on_sites_that_have_the_facebook_social_plugin.pdf )
A FB "Like" button is useful, but site owners should not use Facebook to support comments. Use Disqus, or LiveFyre, or anything else besides Facebook.
(I realize this doesn't solve your problem, sorry! But perhaps if enough site owners dump Facebook for comments then Facebook will stop what they're doing.)