And sometimes you don't. Zuckerburg famously had the chance to sell out for billions quite a few times [1], and basically said, "I'm doing what I want to be doing. If I sold out I would just start another social network company and I already own the best/biggest one."
edit: Closer and better quote. Thiel's paraphrase of Zuckerburg, "I don't know what I could do with the money. I'd just start another social networking site. I kind of like the one I already have."
Are you suggesting they never had a chance to sell for 19 billion? Because I'm pretty sure all Zuckerberg had to do was pick up the phone in 2009, or even earlier.
It's plausible. They crossed $20BB valuation roughly at the end of 2009. At that time they had 400MM users. This is about the same growth rate as WhatsApp since founding.
However, Facebook had 400MM largely North American and European users and had $1-1.5BB revenue with obvious potential for much more. WhatsApp's revenue was a couple orders of magnitude less after five years, with lower potential on its own to grow since it lacked Facebook's ability to collect and exploit data.
So, it seems like WhatsApp should be valued less than Facebook, i.e. less than $20BB, at the same stage in its life. That doesn't mean Facebook overpaid; WhatsApp's users were nearly a perfect complement to Facebook's geographically and Facebook, unlike a company like Google, had the organizational ability to allow WhatsApp to grow while benefitting from Facebook resources.
