That's not true. They were charging $1/user/year (possibly only on Android and in some countries) before their acquisition. And their costs were low. I'm pretty sure they were profitable.
> WhatsApp’s six-month revenue for the first half of 2014 totaled $15.9 million and the company incurred a staggering net loss of $232.5 million, though the majority of that loss was for share-based compensation.
WhatsApp was always running on VC cash.
In my country (Argentina) it became a de-facto platform for selling for small businesses, grocery stores and restaurants. There are a couple of 3rd party e-commerce platforms that generate the order and then simply drop it through WhatsApp so you can continue through there and handle the payment manually.
... There are many ways in which WhatsApp could have explored making profits. Facebook simply made it stagnate.
WhatsApp can actually be monetised successfully by a company like Facebook.