A couple of people have noticed that the Facebook Developer Toolkit doesn't work on the current release of Windows Phone 7. The primary reason is that it has a dependency on System.Windows.Browser - a library which isn't in the WP7 runtime (Silverlight isn't hosted in a browser on the phone).
I took the source from the Facebook Developer Toolkit and hacked at it until I got it to work on the phone. Here is a quick demo showing it working in the WP7 emulator:
(click 'read more' for more on how this was done..)