Don't get me wrong. I like the signal foundation. They do great work. I'm just hesitant to claim that usernames are in anonymous way to talk to people on the internet.

If your model of signal is just I can communicate and encrypted with people I already know and who know me. Everything's fine. Nothing about the server compromises that. But when you introduce can I talk people anonymously, the model doesn't support that. Because the server has the capability to deanonymize.

If nothing else, somebody could simply brute force all the phone numbers, until their named contact shows up.

I just did some minor testing, right now, if you have a username that you want to post on the internet, like embarrassing_contact.01, like for political dissidents organizing, alternative lifestyle organization, disclosing sensitive information etc. if you already have the contact on your phone, but you try to send a message to the username. Signal will tell you oh it's the same person you already have this contact. So right there is a proof of concept of deanonymizing people

https://github.com/signalapp/Signal-Server

Everything is completely open under AGPL 3.0, really neat.

I think you are confused between telegram and signal.

Well, it's big pretty much everywhere except for the US and China. It was the right app at the right time and due to its huge user base it's virtually impossible to get people to ditch it - even when it was bought by an evil tech giant.

It does have good and useful features though. SMS doesn't even allow group chats - that alone renders it utterly useless (I'm just ignoring it being clear text without any kind of security or privacy - those seem to be too abstract concepts for most people anyway).