Honestly my own experience makes me wonder how many of those "users per month" either accidentally invoked it by holding down the button on their phones or had Siri just pop up and start yakking because its hotword detection went wrong.
My company had to switch from Microsoft to IPhones. I hear Siri every day. Usually followed by swearing.
When I got mine, I was trapped in some Siri -> Lock Screen -> Siri combination. "Siri go away" and "Siri show me the home screen" didn't help either. I just wanted to get on the home screen but since there is no Back-Button...well it's disabled now.
