I think it should also be pointed out that for all of the "Facebook is free, you are the product" and that maybe they should charge, Whatsapp had an existing monetization model

Zuckerberg is ruthless and cutthroat and Whatsapp was less a "savvy acquisition" (I mean, yeah, get rid of competition) than a "I want this, I need to own it"

https://news.ycombinator.com/item?id=1692122