... Signal began by using the social graph that already lives on everyone’s phones: the address book. Rather than a centralized social graph owned by someone else, the address book is distributed and user-owned. Additionally, having the social graph already on the device means that the Signal service doesn’t need to store a copy of it. Any time someone installs or reinstalls Signal, their social graph is already available locally.
https://signal.org/blog/private-contact-discovery/
... Signal began by using the social graph that already lives on everyone’s phones: the address book. Rather than a centralized social graph owned by someone else, the address book is distributed and user-owned. Additionally, having the social graph already on the device means that the Signal service doesn’t need to store a copy of it. Any time someone installs or reinstalls Signal, their social graph is already available locally.