"using people's cellphone numbers, provided for 2FA to target them with adverts"
....."even though the numbers were only provided for security reasons rather than ads.".....
And that in one sentence is why 'Acton of WhatsApp fame' bailed... Pretty good inside take on the WhatsApp founders departure below and the rise of Signal as an App. The chilling ruthlessness of Facebook is pretty clear here:
....."When Acton reached Zuckerberg’s office, a Facebook lawyer was present. Acton made clear that the disagreement—Facebook wanted to make money through ads, and he wanted to make it from high-volume users—meant he could get his full allocation of stock. Facebook’s legal team disagreed, saying that WhatsApp had only been exploring monetization initiatives, not “implementing” them. Zuckerberg, for his part, had a simple message: “He was like, This is probably the last time you’ll ever talk to me.”.....