I had Facebook AWD All in one on my blog which was working fine.
I tried to install Comments Plus from WPMU-Dev the other day and Comments Plus threw a fatal error as follows:
"Fatal error: Call to undefined method Facebook::getSession() in /.../wp-content/plugins/comments-plus/lib/class_wdcp_model.php on line 47"
After disabling Facebook AWD All in one, Comments plus installed and activated fine. I've contacted WPMU Dev (click link for the conversation) who have asked that I contact the maker of Facebook AWD All in one as they don't think it is caused by their side of things. They say they are happy to help sort this out.