Umm... in fact "most people" communicate outside of those groups, as that's how dating works, and it is a big reason why Snapchat dominates "private communication": people don't want to give their "real phone number" to someone until they trust them. Signal needs to realize its big competition is not WhatsApp, but is Snapchat (particularly given that WhatsApp is pretty damned secure, and so should be seen as an ally versus all of the actually-insecure messaging apps of the world).