Any source for this data other than telegram PR?

Even if that were truth I know pretty much nobody who has it, while I think I dunno single person who wouldn't have Whatsapp, so I assume majority of those Telegram users are concentrated in few countries lika Russia, Iran etc. or in some specific social circles, which I am not part of.

Steady stream of new people showing up tells me at least there is a heavy influx of users and have been for a long time.

Edit: you might also be onto something with the "specific social circles" part of the comment.

It is maybe like Google+: from the outside it looked like a ghost town, while inside it was beautiful.

>Any source for this data other than telegram PR?

You may go by Google Play Store installations. With about ~1.5 billion installs that's about a third to a quarter of WhatsApp installations. WhatsApp claims about two billion active users, so that seems about right.