UPDATE (12/30/2011) : Facebook Connect bindings are being maintained by the Mono Project UPDATE (8/3/2020) : This information is widely out of date and not current. Facebook Xamarin bindings were moved to facebookios.
The MonoTouch libraries can be found on github.
It’s late and I am tired so I’ll be brief. I just pushed the binding of official Facebook iOS SDK to github. In addition to the bindings, the DemoApp, which was included along side the SDK, has been ported to MonoTouch. The code is straightforward and does a good job of getting you up to speed with the important elements of the SDK.
If you want to run the demo application, you will have to create a Facebook application on Facebook and grab the app id. You need the app id to pass back to Facebook during the login process and you can find out where to put it by looking through the comments of the DemoAppViewController class.
Please let me know if you have any issues with this library. Enjoy.