Keybase might be worth a look. The only complaint I have is that there seems to be no way to self-host it (which is an option I'd like to have.) And the server source seems to be hidden from public which is not great for trust.
The network effect doesn't really concern me. For communication with strangers I'll have to fall back to email anyways since WhatsApp is virtually the only thing that has presence in my area.
Don't use it as much as the others because of network effects, but it works just fine. Requires a user account but no phone number.