[hardwarescope]

I think this issue is more complicated than people realize. There are really only three options here:

1. Use the device's address book (phone numbers).

2. Use Facebook Connect (FB id).

3. Store the entire social graph on the server (custom identifiers).

I think #3 is what every messenger that offers non-phone identifiers does (Snapchat, Twitter, Telegram, Wire, Viber, etc).

The reason is simple: if someone does manage to create a social network by slowly discovering a bunch of usernames from their friends, but then they reinstall the app or get a new phone, it would be pretty unusable if that entire social graph was just... gone. It's bad enough to have to create this social graph from scratch, but to do it every time you reinstall, lose your phone, or get a new device?

The consequence is that many people advocating for this feature (or using other messengers because of it) probably don't understand what it is that they're really advocating for or getting themselves into.

Right now Signal is much more "private" than any other messenger, if you measure that by how much Signal knows about you (timestamp of account creation is the only thing iirc). By supporting a custom identifier, they would have to store your entire social graph, like other less private messengers.

[Terretta]

For a counter example, consider Threema. Granted, it can be a rigamarole if you opt out of publishing contact info. But you can. And then backing up and restoring your graph is up to you, or you can rebootstrap people one by one.

On iPhone, after a hardware upgrade w/ restore, Threema offers to restore your client side graph from a client side backup.

[enimodas]

It can be stored on the server, and encrypted by the users' private key. Like a message to himself.

[hardwarescope]

That doesn't solve the case for when you get a new device or reinstall the app. Any keys you had are lost.