I think this is great news. One thing that looks suspicious is 15 billion messages a day. It must count group messages as a multiple of all subscribers and maybe some other dirty tricks. Can't believe that an average user sends 150 messages a day.
However I'm not sure how common this is and if it could be one of the reasons for the high amount of messages per user.
I think that the web and desktop clients just push this probability. Having a computer keyboard instead of a phone to write triggers a different "state of mind" in the user, I think. At least it's like that for me.
If I talk to two people, over the course of two hours, at one message a minute, that's another 240 (2260) messages.
The numbers seem fairly plausible. (I do think there's a fair number of spam and bot messages included in the figures.)
https://telegram.org/blog/15-billion
So it's push notifications/notifications.