I haven't heard of it either. If it is a messaging system, surely the most important part is who you talk to, rather than the system itself. I guess that that leads onto the question of how popular a given messaging system is among native speakers of the language you want to learn. Hopefully this question is not just spam!

Hi Camilo! Telegram is a Russian instant messaging systems, it's very similar to WhatsApp and Viber. These days, Telegram is the most popular with my countrymen. :)

I have never heard of Telegram.