facebook-csharp-sdk/simple-json — comment on commit d93de45c11c24ec8b3c8fcfbc15afdc6472f81c1
File:  line 0
By Minimalist:

a double has 15-16 digits of precision per http://msdn.microsoft.com/en-us/library/678hzkk9(v=vs.71).aspx

a long can be up to 20 digits per http://msdn.microsoft.com/en-us/library/ctetwysk(v=vs.71).aspx

Therefore I believe this has not fixed the problem. I can't test, but I believe you will start seeing errors with lack of precision on larger longs.