I don't have my uncle's phone number, because I'm in the US and he's in Australia and it's costly for me to call/text it. I keep in touch with him via Facebook primarily.
Be careful conflating "I don't do X" with "no one does X".
Be careful conflating "I don't do X" with "no one does X".
If you were, you'd have his imessage / whatsapp. Facebook is a 3rd tier form of communication for a reason.
FB Messenger is just fine. It has the same features as iMessage and WhatsApp, and plenty of folks use it as primary comms.