Whatsapp itself is not revenue generating. Owned by Facebook, Whatsapp provides Facebook with all the data of over a billion users. Have you chat over whatsapp with your friends over a particular item or topic, and the next moment find related ads appearing on your Facebook feeds? Facebook uses such data and massive analytics to gather info and send targetted ads to everyone. These ads bring in billions of dollars to Facebook.
When Facebook bought Whatsapp for 18 Billion dollars, people were saying FB is crazy. But now, it seems like that was a brilliant acquisition.