It’s probably true that not many people use the IM portion of iMessage in the EU. In my experience, I’ve never actually seen anyone use it.
That being said, since it’s the default SMS app, you can’t convince me that most iPhone users don’t use it al least once a month (even if just to get a one time code from the bank). It’s definitely more than 45 million users, at least for the SMS part of the app.