I work on Telegram only with my team for 4-5 years. Skype became unusable by the time.
Frontend is exactly like whatsapp but instead of green color is blue.
I like that it works even with switched off mobile phone unlike WA. Recently they added video calls and they are better than skype/wa/googla calls. They are Russians, so it is a relatively free country, when compared to Europe or even worse USA and generally I do not care which data Putin collects.
I am just a passive member of some groups, I would never do social marketing now, too painful to talk to normal people. Everyone is lately switching to Telegram, I believe it will be nr.1 instant messenger.
However they also resell data. For sure when something is written there, like cat - you can be sure you will see cat food ads everywhere. And now I will come with a conspiracy, but I am nearly sure Telegram is doing it. They do not spy only on written keywords, but they spy also on the speech. Many times I was not writing some specific topic, I was just saying it laud offline with my computer/mobile phone next to me. E.g. special things like "Cushing syndrome" and immediatelly I saw ads aimed at that special term. It could be some other app sniffing, but I suspect most the Telegram.
But whatever, it is a great platform, just when it is free you are the product.
If you talk to God, you are praying; If God talks to you, you have schizophrenia.