Comments Plus won't activate - Fatal error

Seemed to load fine, but when I tried to activate I received this message. Don't know if it's related to the other issues here or not.

Thanks

Fatal error: Call to undefined method Facebook::getSession() in /home/nitrousc/public_html/notanotheraveragejoe.com/wp-content/plugins/comments-plus/lib/class_wdcp_model.php on line 47