I wonder what markets WhatsApp is important to. In the UK quite a few people used it while briefly lived there, but in the US, Thailand, Vietnam, et.al. I didn't see anyone using or ever talking about it.
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.