Ever since its launch, WhatsApp has had two business models on paper: you pay for the app on iOS or Blackberry (that has changed a long time ago when the app went free) or you get the first year free and you have to pay $0.99/yr afterward. However, that latter model was never put into action consistently. Despite having used the messenger for more than 6 or 7 years, I was never charged once and neither have any of my friends and relatives. Whenever the expiration date approaches, we get a small pop-up telling us that our service has been extended for another year and that'd be it. But that doesn't mean that other people weren't being charged.
That didn't work out well for WhatsApp, first because of the inconsistency of the business model and second because of users' fear of being locked out for lack of a credit card or other payment method. So the service has officially announced that it was going completely free starting today. All of WhatsApp's apps across platforms will be updated to reflect this in the next couple of weeks. Unfortunately, if you're one of the unlucky users who were asked to pay and did so recently, you don't get a refund. Ouch, I hope you didn't go for the 5 years plan.
To stave off fear of the platform becoming infested with ads like its parent company Facebook, WhatsApp is quick to point out that there won't be any ads. In order to stay profitable and give back to Facebook some of those $19B it paid to buy the service, WhatsApp will be investigating better ways to stay in touch with the businesses and organizations that you choose to communicate with (read: not spam). Think banks, airlines, stores, restaurants, and more.
Right now, in Lebanon for example (and many other countries I presume), all of these send their automated messages frequently through SMS and sometimes through email. Since WhatsApp benefits from a high penetration rate in many countries, it could easily offer a paid plan to businesses to be verified and automate some communications with their customers. I'd sure prefer that over SMS and it could be much cheaper for businesses to use.
However, WhatsApp isn't really clear about how this could effectively work. For now, it's simply testing the idea, so the business plan is basically as clear as this: