That stayed true for a few years, and so WhatsApp became the de facto standard for messaging.
I can’t speak for other countries but in South Africa that’s why. And the rest is just networking effects.
My phone number is an ID. An ID I don't want to share. An ID that can be stolen by calling the phone company and saying you're me and bought a new SIM.
Most people who meet each other in real life and want to exchange contact information exchange… you guessed it… a phone number. Maybe you exchange email or twitter handle, but again you are an exception here. So imagine we meet for the first time and we exchange phone numbers. Now I can WhatsApp you, just like that no other account info needed.
It seems to be available on cheap feature phones now anyway: https://www.gizbot.com/mobile/features/fnord-047499.html
Whatsapp grew big with customers of telcos like Vodafone, which may be less capable than Mark Zuckerberg's snoopshop but I wouldn't bet on their being nicer. See https://toroid.org/vodafone-smtp-mitm for example.
https://www.gadgetsnow.com/tech-news/Indias-cheapest-Android...
And there's a fair choice for under 1500INR
Now sure you can get a non-whatsapp phone for half that price, but it's pretty cheap
-you download the app and can immediately get started after verifying your phone number (no ID or password required) -you don't need to share any ID or connection details other than just your phone number
I spend a lot of time in India, and I think this lack of complexity has contributed significantly to its virality (I'd estimate that a pretty significant percentage of the user base does not have or regularly use an email account, which is usually a prerequisite to setting up many accounts).