You're partially correct. Whatsapp rose because carriers didn't count any whatsapp payload towards your metered bill. Basically, Whatsapp was virtually free-of

You're partially correct. Whatsapp rose because carriers didn't count any whatsapp payload towards your metered bill. Basically, Whatsapp was virtually free-of-charge whereas any other messaging app/system (SMS included) incurs towards your data cap. At least that's what happened (And still happens) in Mexico.