Whatsapp entered a highly competitive market. They've got the highest active users in the SMS market, and over 400M subscribers within 4 years. How they did it?
They've kept it simple, you don't need to fill up sign up forms and remember your user name. Your GSM SIM card is your identity.
Secondly, they tightly focused on SMS market. Didn't go for multi party video conferencing, screen sharing, and other seemingly cool stuff.
Majority of adoption was from oversees. Also it was used in a few large companies. The major reason was due to cross-platform solution on all mobile OSies. Applies text platform did not work well with Blackberry/Android/Windows