iMessage shows up occasionally while forming initial connections, but I tend to move the conversation to one of these other services as soon as possible and the other party is almost always OK with that. (I do this in part because I don't regularly use my Mac and these other programs have web versions that can be used on all platforms including Windows PC.)
If you're curious about demographics I'm in my mid 30s. My particular social group is not big into Facebook or Facebook Messenger (though I know they own WhatsApp) since we are techies and privacy-conscious. I'd rather have a 1on1 or small group chat than blast thoughts out into the ether when in that mode of communication. (Hacker News is different. It's cultivated, moderated, and oriented around exploring curiosity.)
My personal pet theory is that it depends on when the country got free text messages. For example in Sweden everyone already had unlimited texts before smartphones were ubiquitous, therefore WhatsApp never really had a purpose and Messenger became the supplementary app.
[0]: https://www.similarweb.com/corp/blog/popular-messaging-apps-... (Not sure about the source but the countries I know are reflected correctly.)
Asia is LINE/WeChat. Very few use either FB or Whatsapp.
This is...not true. Line has Japan and Thailand, WeChat has China and the mainland diaspora, and the rest is mostly dominated by Messenger, WhatsApp, or a local app (e.g. Zalo in Vietnam or KakaoTalk in South Korea).
Source: me, a person who has lived in Asia for a long time, and the bulging folder of messaging apps on my phone.