Facebook Messenger - approx 140 million users, then WhatsApp at approx 75 million. iOS has approx 136 million users (not sure if that includes iPad). So "dwarfs" might be a bit extreme. However, its extremely unlikely that all the iOS users use iMessage and none use either Facebook or WhatsApp. Statista has the figures, but I'm not going to pay $149 per month to find out more!

Source: Googling around, so take it for what it is!

Many Americans have Facebook/WhatsApp accounts they haven't used in years. I'd be skeptical they surpass iMessage in terms of volume of messages

It's not about having accounts. Again, the data source (which I admit could be wrong/misleading) all say active users, which I take mean users actively sending messages using FB messenger. The sources, by the way, are companies that sell services that market to consumers using these services, so it's in their interest to know usage stats and patterns. Admitedly finding information around iMessage usage is harder, which is why I went with number of active iOS devices. Yes, it's sketchy AF, but I'm not going to pay Statista $149 in a vain attempt to pyrricly win internet points.

With this numbers I would still guess that iMessage has WhatsApp beat.

I would also guess it does more volume then FB Messenger, even if it technically has less users.

How? 140 > 136! And the figure reported are active users. I accept it's back-of-the-napkin, not trustworthy sources, but even so, your math just doesn't make sense.

I'm just speculating that iMessage users use the app more then FB messenger users use FB messenger. I don't think there's anyway to know, so I definitely might be wrong.