I'm guessing most people didn't pay though, since they scraped the fee (even before FB bought them). I guess it was just too little money to be worth the effort.
The fee wasn't enforced in many developing countries, and some users elsewhere will have been jumping through the delete-and-reinstall hoops (which was painful because it lost chat history) to avoid paying.
But with 1bn active users at the time the fee was dropped, it would still have been bringing in more than enough revenue to have sustained Whatsapp as an independent business if they had chosen not to sell to FB.
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-...
So I think I got that...