Signal has an interesting article describing how, while contact discovery allows users to populate an instant social graph, Signal's servers still do not have access to that information, even as they send "so-and-so is now on Signal!" notifications:
Of course, they have the ability to push a new client that hoovers up whatever they want, especially with their time-bomb policy of preventing old clients from sending messages until they're updated. But I was impressed by the lengths that they go to to build this privacy-preserving contact discovery service. I was especially interested to see their use of remote attestation "for good" and to preserve privacy and freedom, rather than systems like DRM and WEI that seek to compromise those.
I meant in Telegram you don't need to share your phone number with your contants or members of a public chat you've joined. While in WhatsApp or Signal your contacts know your number and can easily find out your identity and where you live if you write something they don't like.
https://signal.org/blog/private-contact-discovery/
Of course, they have the ability to push a new client that hoovers up whatever they want, especially with their time-bomb policy of preventing old clients from sending messages until they're updated. But I was impressed by the lengths that they go to to build this privacy-preserving contact discovery service. I was especially interested to see their use of remote attestation "for good" and to preserve privacy and freedom, rather than systems like DRM and WEI that seek to compromise those.