Earlier quoted context omitted.
As of today: Smartphones outsold feature phones for this first time ever in India. http://indiatoday.intoday.in/story/samsung-smartphones-outse... From my personal experiences, international countries that have smartphone adoption use Viber, and the feature phone countries use WhatsApp.
WhatsApp is also really strong here in Germany and other parts of Europe. More of my friends are on WhatsApp than Facebook (only very few more though).
It has basically replaced texting. Even my mom use it, which for me it's the definition of "wildly popular".