Holy shit, that's a lot of money. And it sounds like the deal got done in a hurry. Here's The Verge's coverage of the buyout announcement from earlier today.

Some key figures:

WhatsApp has 450 million active monthly users
WhatsApp launched in 2009
Very small staff at 32 engineers
Facebook paying $16 billion in cash and stock, plus an additional $3 billion in restricted stock to WhatsApp app employees, vesting over 4 years after the deal closes