Unless you can prove without a shadow of a doubt where the key came from this is a ludicrous idea. My key actually comes from Green Man Gaming and I have had it for a while. I know others that have been sitting on some keys for a while as well. Yes there will be some who try to make money off the current FB gimmick, but I would suspect those people would be in the minority.
Ban or not, it's going to be next to worthless regardless. First, we had the Amazon bundle. Next, humble; now, Facebook. I've been sitting on a tradeable version for a while and I know I won't be getting anything for it.