The official WordPress Facebook plugin no longer shows any of my fan pages. I have tried removing the app information and then uninstalling the plugin, re-installing it and going through the set-up process again. It seems to work ok but when I try to authenticate by clicking on one of the ‘Link your Facebook account to your WordPress account’ links on the add post page, a pop-up appears for a few seconds, but disappears without actually displaying any information, and there are still no fan pages listed in the drop-down box.
WordPress Facebook plugin no longer showing fan pages
Probably the plugin has a outdated code. I recommend you to go to Facebook docs and generate your own code and put it on your HTML without using a plugin: http://developers.facebook.com/docs/reference/plugins/like-box/