Pre acquisition Whatsapp had 450M users. Even accounting for half the revenue of 1$ going away for payment fees (30%) and taxes (20%), that would still have been a nice cushy 200 million $ a year in almost pure profit - WA had 55 (!) employees at acquisition and 550 servers [1].

That's nothing at this scale of users and speaks volumes for the ingenuity of their staff.

The only ones driving even leaner than that are StackOverflow with just nine servers [2].

[1] https://highscalability.com/how-whatsapp-grew-to-nearly-500-...

[2] https://news.ycombinator.com/item?id=34950843