Sucks to be working hard on your Tamagotchi armadillo + tumblelogging viral Facebook mashup and see:
Official documentation is here.
As usual, it lives in the Fauna. Install via Piston, or otherwise the old-school way:
script/plugin install -x svn://rubyforge.org/var/svn/fauna/facebook_exceptions/trunk
No tarballs/gems yet. Requires edge Rails, probably.
The support forum is here.