There are a bunch of unconfirmed transactions right now so maybe the withdrawal is just stuck. You can wait for some time or maybe you have just been scammed. Maybe there are others as well who are in the same boat as you so you can meet altogether and plan your actions. If you can't do something about it then this transaction will just served as a lesson. I have been scammed before as well for more than one bitcoin in another site and wasn't able to get it again.