I find it interesting to that people don't use WhatsApp at all. Many posts here are claiming not using WhatsApp is impossible. Grandparent's experience demonstrate that it is.
Most Americans, for example, have no need for it. I feel like iMessage is the dominant messaging platform in the U.S.
That diagram is for Android only, so I wouldn't read too much into it.
Edit: US - sorry I was just catching up on the rest of this giant thread and it looks like SE Asia S America are the main usage places.