They didn't pay it. It was mostly just testing on a subset of US users.

I didn't say people are not willing to pay money to use an app that has free alternatives. I said good luck finding 2 billion such people. Whatsapp today is used by everyone. 8 year old kids and 85 year olds. The network effect. Its biggest asset is the number of users which makes it easy to communicate with everyone you know on it (not everywhere but in many countries). If they charged money it would've been different. It's not about the money. Everyone can afford $1/year. Even kids in poor countries. But not everyone has access to a credit card for example. Many wouldn't bother and use a free alternative like FB messenger or Telegram or whatever.

It was this popular in 2012 already. For Android users first year was free and then you could be charged €0.89/year. I knew paying customers and was willing to do the same. It's a great product at a great price without ads, just take my money. I'd rather pay for a service than be monetized in exchange for using it.

BTW, they had just 50 engineers when FB bought them with 450M users (+1M/day) and they supported more devices back then. Just with those costs and the network effect, keeping the subscription model would have meant a solid source of honest revenue.