In this particular case, I can almost guarantee that the reason the deal ended up this huge is that the founders absolutely, positively didn't want to sell, and

In this particular case, I can almost guarantee that the reason the deal ended up this huge is that the founders absolutely, positively didn't want to sell, and especially not to facebook. And facebook had to literally make them an offer they could not refuse (in the sense of the amount of money for every employee... not as in the Corleone sense). You can glean some idea of why that was from this public information https://blog.whatsapp.com/245/Why-we-dont-sell-ads https://blog.whatsapp.com/245/Why-we-dont-sell-ads. This is in direct voice of Jan Koum, the founder and CEO of whatsapp. That gives you some idea of his personality and it's an entirely accurate one. I'll be willing to bet large amounts of money that the Zuckerberg himself had to engage in a full court press before getting a response at all.