I installed Telegram yesterday. The reason was pragmatic. I need to coordinate with Vera about a theatre trip while Lee is out in the North Sea on a rig. She sent me an SMS message from a French (+33) number but I couldn't reply to it for some reason. She isn't on WhatsApp so he advised me to try Telegram instead. It is all up and running now.
I was vaguely aware of Telegram before, having heard references to it in relation to the Russia Ukraine conflict, and by coincidence there was press coverage yesterday saying:
The private messaging app Telegram has for the first time overtaken WhatsApp in traffic volume in Russia, the Vedomosti business daily reported Monday, with experts predicting the Russian-founded messenger will also surpass the user count of its Meta-owned competitor this year.
Telegram accounted for 60-80% of total traffic exchanged in Russia by the start of 2023 and has continued to grow since then, according to an analysis by Russia’s top four telecom operators cited by Vedomosti.
Here is the Wikipedia page which seems thorough for all that it is mostly technical.
Here is something I picked up from ITV News on YouTube.