"The device’s address book is now the social network, so using phone numbers as an identifier has reduced switching costs by putting a user’s social network under their control. In a way, the notification center on a mobile device has become the federation point for all communication apps, similar to how older desktop IM clients unified communication across multiple IM networks."
https://signal.org/blog/the-ecosystem-is-moving/
"The device’s address book is now the social network, so using phone numbers as an identifier has reduced switching costs by putting a user’s social network under their control. In a way, the notification center on a mobile device has become the federation point for all communication apps, similar to how older desktop IM clients unified communication across multiple IM networks."