UPDATE (12/30/2011) : Facebook Connect bindings are being maintained by the Mono Project
I pushed some updates to the monotouch-facebook project recently to eliminate potential linking problems that could come up. Apple introduced a bug in their linker that causes problems when linking non-Thumb MonoTouch code with Thumb code. The default settings for the Facebook iOS SDK project will produce a static library with Thumb code so I forked the project and turned off Thumb code generation.
I also made the fat static Facebook library a little fatter by linking the simulator version of the library (i386 architecture) in along with the iPhone and iPad library (armv6 and armv7 architectures). This fatter library doesn't require managing and maintaining two versions of the library for the different architectures.
The bindings have been refreshed due to the Facebook iOS SDK being updated recently. You can see the changes reflected in the example app. The biggest change introduced by the update is the app's Info.plist now needs to be modified to register a custom URL Scheme that will enable the example app to be called back after the user authenticates with Facebook. You can see what the Info.plist should look like in the picture below. You will need to replace <<<YOURAPPIDHERE>>> with your real app id.
I hope to get this info up on the monotouch-facebook project's wiki page soon. If you find any issues please open them on the project's issue tracker. Enjoy.