This is meant do to one thing -- let the founders retain control while allowing for large stock acquisitions. It's possible Google was muscled out of the Whatsapp deal simply because they couldn't pony up the money. $20B is almost half of their cash on hand. Note that the Nest and Waze acquisitions (small in comparison) were paid for almost entirely in cash.
Did FB pay $19B for Whatsapp in order to prevent Google from acquiring their userbase? That is, FB got into a bidding war with Google over Whatsapp?
$19B is a whole lot of money, even (especially?) for a company the size of WhatsApp