Facebook only gets screwed if their own valuation doesn't increase enough to match the amount "overpaid" in response to Instagram's acute "increased valuation" ploy.

Given current market conditions, particularly in terms of fawning over Facebook, the negative effects of this ploy seem likely to be largely externalized, with the amount of burden borne by Facebook promising to be negligible. As usual in the trading game, the bulk of the burden will fall on the unanointed schmucks lacking the information to buy low and sell high.