I am questioning one thing. How did Apple's iMessage get so many users?
its a default messaging app, dont need to download it
Other than just being default messaging app
In what way? With existing iPhone users or with people getting their first phones and then becoming an I message user for life?
With existing iPhone users.
Default messaging app, prompts you to sign up. Also on the iPhone you cannot change your SMS app, so people have to open iMessage.