How does companies such as WhatsApp, Twitter, SoloLearn make their money? In that they dont run adds

Question

No adds on their application.

Answer 1

Answers will help much than up votes. But thanks for thumbs up

Answer 2

I found a discussion about theories how SoloLearn makes money here:
https://www.sololearn.com/Discuss/158386/how-does-sololearn-make-money-theories