Also, Telegram is not private.
1. It's not E2EE by default
2. It's not E2EE for groups on any platfrom
3. It's not E2EE 1:1 on desktop clients forcing you to downgrade from secret chats to insecure chats
4. It's collecting 100% of your metadata, including
* who you talk to, when, how much, what type of data you exchange,
* your IP-address which sort of defeats the purpose of having no phone number, and
* when you enable secret chats
Telegram is also not transparent about its funding, about who develops it, and who has access to the plaintexts stored on their server (meaning, anyone with a zero day or two).
Journalists who went to look for Telegram's office in Dubay found out no-one in the neighboring office had ever seen Telegram staff enter the space https://www.youtube.com/watch?v=Pg8mWJUM7x4
Telegram was built with blood-money from VKontakte, and Durov has been marketed as living in exile, when in reality he has visited Russia on average once every 2.4 months since the exile began, and strangely Durov has not had his underwear poisoned and windows have been kind to him despite supposedly betraying Putin's interests.
tl;dr Telegram reeks of FSB/SVR honeypot.