Behind the development of applications is a company with only 50 employees. According to the company's founder, they achieved such great success and popularity without any advertising. The popularity of applications is earned simply by its ability which allows users to exchange messages between themselves only at the expense of mobile data, which is quite small if you just use text messages.
Unlike some other messaging services that like to spam you with ads, or charge for their services, WhatsApp decided to stay away from that kind of business. You can try it for free on a period of 1 year(!), and after that you need to pay only 0.99$ per year in order to continue using it. And from personal experience that fee is nothing if you compare it with the quality of service which is great.
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