Unless this was some special partnership, then Facebook will have a contract with a clearly stated MOQ (Minimum Order Quantity).
If they cannot make that commitment to purchase from HTC the amount stated then HTC will charge them to get out of the contract. This will be broken down into costs for the handsets already produced and unsold, materials already bought and some additional fee for lost revenue on the rest.
In short, HTC will be okay (they'll even make a bit of profit, although not as much as if the phone had been a success) however it will be Facebook who will be left with one expensive bill.
(been there and, sadly, done that)