I listened to a talk by Jan Koum from WhatsApp right before the acquisition by Facebook. His key point was that WhatsApp success comes from the fact that they only do messaging (as an sms replacement) and keep the app simply. He was talking about a lot of functionality that he did not want to add (desktop client, video chat, games, anything commercial or e-commerce related) and said WhatsApp wants to keep the surface are of it's app and developer team as small as possible. I don't know how much that changed after the acquisition, though.