In Europe: WhatsApp, no one uses SMS anymore because WhatsApp started getting popular when people still had to pay for each SMS sent.
I currently live in South America, same story: Nobody uses iMessage, everybody is on WhatsApp. Even businesses prefer WhatsApp over phone calls.
The biggest difference between Whatsapp users in Europe and South America that I observed has to be the crazy amount of people using voice messages to communicate. Why don't you call and speed up the conversation by 50%?
That's not true, it may be true for some parts of Europe, but other countries had free or extremely cheap SMS before WhatsApp even existed.
Depending on where you are in Europe SMS may still be vastly more popular than WhatsApp. It also depends on the audience. Most of my family still use SMS. Technically it's iMessage but they don't know the difference, and they certainly don't care how messages are transferred. The build in messaging app is what is used.
Personally I've seen WhatApp installed only by people who have a large number of friends in eastern European countries.
Edit: Just to be clear, Denmark have had the one of the most competitive markets for cellular in the world for the past 20 years. Prices have been driven ever downwards for the last 15 year. So calls and sms have almost always been cheap.
It is asynchronous communication, like emails, the recipient can listen to it when they are available, but you don't need both to be available in the same time.
I currently live in South America, same story: Nobody uses iMessage, everybody is on WhatsApp. Even businesses prefer WhatsApp over phone calls.
The biggest difference between Whatsapp users in Europe and South America that I observed has to be the crazy amount of people using voice messages to communicate. Why don't you call and speed up the conversation by 50%?