How do Apps (Whats up, Viber, Wechat etc) software development companies generate income as most of them do not sell advertisements.

In the social media era, you can come with a brilliant innovative creative idea. Then make it public as much as you can (essentially by making it free of charge for the users). After your product (i.e. your app) goes viral, hot, and very public; then you can wait for any larger company to buy it (hopfully, FaceBook)! or, you can then start sell ads, offer free-ads in-app purchase for the users who then would become addicted to it.