Whatsapp was not a "instant messaging" app in the way you put it. It was started specifically to get around high SMS carrier rates in countries around the world

Whatsapp was not a "instant messaging" app in the way you put it. It was started specifically to get around high SMS carrier rates in countries around the world, hence their ultra-fast adoption in ROW. Not trying to nitpick, just pointing out that Whatsapp didn't start in a crowded market. They actually did something unique that made messaging their friends & family way cheaper and easier.