The only people I know who don't use WhatsApp are very privacy focused and therefor use Signal.

More roughly, my understanding of the messaging space was:

- USA: Facebook Messenger

- Japan, Taiwan: LINE

- China: WeChat

- Everywhere Else in the World: WhatsApp

And the comments here seem to basically bear that out.

We actually:

- have been friendly to 3rd party clients developers, like Loquim (https://loqui.im/). Once WA turned on e2e encryption, the situation for 3rd party clients changed from "difficult but fun" to "mostly impossible".

- wrote a JVM in JS to run the S40 version of WA (https://github.com/mozilla/pluotsorbet).

- partnered with a company specialized in bringing android apps to other OSes (they had Windows Phone support for instance).