Earlier quoted context omitted.
Please stop advocating for Telegram: https://gizmodo.com/why-you-should-stop-using-telegram-right... https://twitter.com/tqbf/status/987372998935105539 Signal is definitely the more secure choice, but Whatsapp is still king of the network effect at least in my neck of the woods.
Update 8/31/2019: After publication of this article Telegram’s creators implemented several changes that are recognized by qualified cryptographers as a vast improvement to its encryption scheme. Since 2017, for example, the MTProto protocol has been recognized as IND-CCA secure.
Probably 99%+ of conversation on Telegram are visible to the Telegram company.