Of course they can afford this. Facebook has currently a market cap of $173.35B (https://www.google.com/finance?q=FB)

They spent on WhatsApp $4B (cash) and $(12+3)B (in stock). $12B is nothing when you have $173B.

If the $173B evaluation is correct, it's a totally different story. But as long as people think it's correct, Facebook can profit and buy any competitor with investor's money.