If this were an all stock deal, and the market cap had fallen by $12B (assuming $12B stock given out), that would mean the market were valuing WhatsApp at $0 as

If this were an all stock deal, and the market cap had fallen by $12B (assuming $12B stock given out), that would mean the market were valuing WhatsApp at $0 as an asset. If facebook had fallen in value by more than the purchase price, that would mean the market is saying WhatsApp has negative value, even if it had been given to facebook for free (though it could also mean a loss of faith in facebook leadership). Any loss in market cap following the acquisition means the market is valuing the deal as having negative net value to facebook.