It doesn't appear Facebook Commenter's plugin works anymore, because Facebook has a new JavaScript API.
I'm not well versed in git, but I created a little write-up on what I needed to change:
http://www.teshack.com/2011/02/updating_movable_types_faceboo.php
Reported on Movable Type 5

Hello Erik,
Thanks for sharing this information. I created a Fogbugz case to review this issue.
http://bugs.movabletype.org/default.asp?105616