By now, most people in Europe also have Messenger, but still mainly use WhatsApp (slightly generalising). I think that primarily comes down to WhatsApp being a much better messenging app. It's quick, reliable, easy to use, power-efficient, small, well-integrated into the OS, secure, and most importantly: media sharing just works.
We also have a low WhatsApp adoption rate. Lately fb-messenger is winning territority (and so is snapchat, but for other reasons), since it offers good group chat on top of a service that already covers north of 80% of the entire population.
In which country? Because I have tons of messengers installed and so do most people I know outside the US.
Sure, I have messenger and hangouts and telegram as well, but WhatsApp is like a primary mode of communication.
WhatsApp on the other hand, is more trusted than Email - When some friend sends me an email, he asks me via WhatsApp if I revived it.
Group Chats, especially with younger folks, are numerous and used for anything, even though the technology is obviously lacking, when compared to other platforms.
But most people either don't care, or perceive these benefits as less important compared to the network effect that WhatsApp has - "I don't want to switch between [random messaging app] and WhatsApp, why don't you just accept what everyone uses".
It's practically the same as with Facebook in the US, and just as bad.
I was in Germany two weeks ago and I tried to get my colleagues on chat...any chat...when I mentioned WhatsApp, they all looked at me like I had a unicorn horn growing out of my forehead. So far I have been unsuccessful in getting any of my team on a chat platform on a regular basis. My company supposedly endorses Lync 2010 (yeah, I know), but probably less than 10% of my coworkers use it and the entire division in Germany doesn't even have it installed on their PC's. Our Lync is also broken by the fact that corporate IT doesn't allow it on mobile platforms.
But this mostly shows that the popularity debate is moot - you tend to know/use what your circle of friends does disregarding of statistics of this article posted.