Found a few of these in my event log:
Exception type: FacebookUnknownException
Exception message: An unknown error occurred (out of memory)
Seems the problem is that one of my users has 800+ friends and when she goes to my invite page the FBServce.GetFriendsNonAppUsers() call receives that exception back from Facebook because the result set is too large (see this thread for more info http://www.facebook.com/topic.php?uid=2205007948&topic=9996).
Can you change .GetFriendsNonAppUsers() to allow paging to reduce the size of the result set? I can say from experience that even users with up to 544 do not cause this error.