I can see how Telegram could be a tool for communicating with specific user groups or something. A closed environment where customers can discuss stuff amongst themselves and with the company. I wouldn't call it advertising, but as a customer engagement tool, it can make sense for companies that want to grow a community. There are tons of tools being used for similar purposes whether it's dedicated forums, closed Facebook groups, Slack, Discord, Whatsapp, etc, but I've not heard of it being used in a B2B sense, I've only ever encountered it in a B2C sense.
Personally, for secure instant messaging with group functionality, I use Signal.