The Facebook Comments for WordPress plugin was last updated in April 2011. It includes Facebook Comments Box social plugin markup that has not been the proper way of integrating the Facebook Comments Box social plugin since March 1, 2011.
Facebook Developers blog post from March 1, 2011 announcing the changes to Comments Box.
The Facebook plugin for WordPress includes the post-2011 version of the Facebook Comments Box social plugin and related markup. Your site and its currently installed plugin uses an xid parameter generated by the plugin to uniquely identify your WordPress post on Facebook and group comments by that generated identifier; it also specifies a migrated parameter to try to live in the world of February 2011. The modern version of the Facebook Comments Box uses your post’s permalink URL as the unique identifier and includes extra features such as moderation.
The bad news: compatibility wrappers are tricky to maintain for a platforms such as Facebook but are typically included to give sites some breathing room to transition to a new system. Facebook has maintained the compatibility for 2 1/2 years so far but it would not surprise me if the compatibility wrapper and the newer system introduced on March 1, 2011 became the only supported way of displaying the Facebook Comments Box social plugin.
The (maybe) good news: a quick search for terms such as facebook comments xid migrated shows some attempts by other sites to migrate comments from the old xid system to the new post permalink system. There may be some tools out there to help automate your transition.
Getting onto the modern Facebook Comments Box is the best path forward for conversations on your site.
