[cocoa19]

Probably in a developing country. Most of latin america uses WhatsApp, a result of carriers charging a lot of money (compared to median salary) for SMS/MMS, while WhatsApp has always been free.

When I came to the US (mid 2010's), I was surprised a large number of people never even heard of WhatsApp.

[stinkytaco]

In the US they use Facebook Messenger, Google Hangouts and iMessage for sure (though not WhatsApp as you point out), but the only thing everyone is sure to have access to is SMS.