Except Telegram is not pervasive in emerging markets like Whatsapp was, and history has demonstrated that users are very fickle with messaging apps - they aren't keeping an essential history like in social media that they cannot easily export or take with them, they are used for moment to moment conversations that are discardable, just like the apps that host them.
Even then, all that argues for is the value in buying Telegram, not in giving them more money. What the hell does an already established and functioning group chat / voice calling / semi-encrypted phone and electron desktop app need in 2018? Whatsapp was spending hundreds of millions just to acquire users with no monetization strategy for the sake of having users, same with Telegram.
I highly doubt Facebook can get any reasonable conversion ratio from Whatsapp user to Facebook regular that already wouldn't happen on its own. Having an audience is worth something, but it is not worth billions unless you are actually getting substantial value from that audience consistently.
https://www.expressvpn.com/blog/whatsapp-vs-telegram-encrypt...