That's weird because it's really a convenient chat app and unlike WhatsApp, works equally well on both phones and desktops (Whatsapp works great on phones but desktop client sucks, Skype in it's old days, worked fine on desktops but totally sucked on phones, Telegram was the first app that did both well).

In our company all of the work chats are on Telegram although it's 100% legal, audited, and does nothing wrong. It's just the most convenient way to communicate. We use Slack only when some client heavily insists on it.