When Whatsapp was bought by Facebook noone understood it. They were offering Facebook Messenger at the time.

It happened, at least that is what people at the USA were telling, that while Messenger ruled in the US, Whatsapp ruled in Europe.

So, no, Whatsapp is not the most popular messaging app

Whatsapp is still the world's most popular messaging app

On the other side, while Facebook advertises F8 as its own Developers Conference, what they do is talk only about their own apps and repeat year after year the exact same things about what developers can find on their eco-system.