Just downloaded the facebook c# sdk. Haven't written any code just added my references to the dll's and ran the project, as I already had some other code I was working on. Then I got this error above. I've definitely added that dll as well as the others so don't know what's happening. I am also using mefcontrib.
If you are using .net 3.5 or lower then you need to install code contracts from MS http://msdn.microsoft.com/en-us/devlabs/dd491992 else solution from above post should work
-Imran