I was closing Mosaic after adding the Facebook tile and signing in successfully, and recieved this crash. I closed the VS error prompt with the 'Message', but it was something like OAUTH exception - you cannot access the facebook api when ...
 
I attached the call stack below. I'll try to get the source and see if I can repro.
 

FacebookApi.dll!Facebook.FacebookClient.MakeRequest(Facebook.HttpMethod httpMethod, System.Uri requestUrl, byte[] postData, string contentType) + 0x213 bytes

FacebookApi.dll!Facebook.FacebookClient.Api(string path, System.Collections.Generic.IDictionary<string,object> parameters, Facebook.HttpMethod httpMethod, System.Type resultType) + 0xf0 bytes 
FacebookApi.dll!Facebook.FacebookClient.Get(string path) + 0x1e bytes   
Social.Base.dll!Social.Base.SocialProvider.GetFriends() + 0x96 bytes    
Mosaic.exe!Mosaic.Windows.PeopleHub.SocialProviderSignedIn.AnonymousMethod__1() + 0x34 bytes    
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool ignoreSyncCtx) + 0xdc bytes    
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x3b bytes    
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x4d bytes   
[Native to Managed Transition]