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