Facebook bought Whatsapp in a brilliant move not unlike Google purchasing Youtube. At the time, Whatsapp was foreign to me. I started using it occasionally with my brother since. However, my default messaging applications in the Bay Area remained simply SMS and Google Hangouts.
During this trip I was amazed how Whatsapp is the single most used mobile mapplication in Argentina. Everybody is using it. Always. Everywhere. On any type of device (although Samsung is the most popular Mobile phone brand in Argentina).
The single biggest reason people started using it was that is was cheap. It works great on Wifi, and does not cost you anything. When using the mobile networks, it uses mobile data, and is cheaper then metered SMS messages.
Secondly, Whatsapp allows you to easily discover your friends and family, and create groups: grupo de hermanos, grupo de primos, los 4 fantasticos, etc.
Lastly, The fact that Whatsapp did not require you to buy into a particular ecosystem (Google, Yahoo, Apple) or required a specific phone, was huge to get started. Everybody with a basic smartphone is able to participate.
It does pay to leave the bubble of Silicon Valley and the US from time to time.
I have changed my default messenger to be Whatsapp now.