Fair point, I should have provided some sort of link. See the graph showing Top Messaging Apps by Country. South America, Africa, Asia is mostly Whatsapp. If you combine FB Messenger and Whatsapp, then FB controls most of the messaging market share in most regions, with a few notable exceptions like WeChat in China.
From the second link it appears that iMessage is not counted because it is preinstalled on iPhones and is not cross platform:
"4. iMessage
Communication apps like WhatsApp are available on practically all devices, whereas native apps like Apple’s iMessage (and now Apple Business Chat) are limited to one provider.
However, the user base for iPhone is constantly growing in the USA. A lot of adolescents prefer iMessage to apps like Snapchat in order to reach out to their friends.
Since iMessage is a pre-installed service of Apple, there are no official messaging usage statistics as they keep those “in-house”. Looking at the ever-growing demand and distribution of iOS devices though, we can assume that the pool of iMessage users is growing. In the fiscal year of 2017, Apple reached a quantity of 216.76 million iPhones."
https://www.similarweb.com/corp/blog/mobile-messaging-app-ma...
And as an alternative source:
https://www.messengerpeople.com/global-messenger-usage-stati...