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Maybe everyone that you know. Surely Facebook has the data to make a distinction like this.

They may not have that data in a form useful for comparison, and the data may conflict with your assumption. Intra business politics / desired integrations / engineering reasons can dissuade teams from a pure data-based decision.

Everyone I know uses Messenger for group conversations.

Anecdotes can go both ways. As far as I've been able to tell, WhatsApp is most popular outside of the US.

I think that's because of WhatsApp's availability as a Java phone applet that was cheaper than texting in a number of places.

Not for Brazil, at least. Here, most people use WhatsApp: local and even larger business are starting to list their WhatsApp number in advertising, even radio stations use it for users to send messages instead of calling.

And it's even hard to spot a Java-applet based phone. Even the lowest classes just use cheaper or older Androids.

for their friend group outside the US