In my observations, it's not quite the absolute majority, but a simple majority of at least 35-40% of people use the mTicket app on the trains I ride.
As of now, there is not any way of getting location services info from the app (on iOS) & I don't believe the activation info is sent in real-time back to a server, so you wouldn't know what train or time that the person pre-purchased a ticket for would actually use it. You could get the data of station-pairs for people that purchase tickets, assuming that those are the actual trips taken. You would miss the monthly mTicket trips.
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Avatar: An overnight trip on Girard Ave. stumbles upon 6 PCC's and an LRV stuck within two blocks.