They are the one example of messaging done right on the client side.

And it maddens me that their whole team, including ops and backend is like 50 people.

At the same time, they used some pretty unconventional ways to find brilliant people. For example, for iOS and Mac and android apps, they simply held a contest for the best open source app fitting succinct and reasonable requirements. The winner got sizable prize and place on the dev team.

Then they released that app as a Telegram X and then gradually sunset the older app.

Pretty neat on multiple fronts.