[_tulpa]

Either this or wire seems to be what people suggest.

How do you convince people to use one of them though?

[DoreenMichele]

"Sorry, I don't have FB messenger. The best way to contact me is X, Y or Z."

Then accept that you will never hear from some people again. And shrug.

[Artemix]

Signal supports "unencrypted SMS" alongside the ciphered messages. You can then use Signal as a central client for both SMS and Signal chats

[_tulpa]

SMS is what everyone abandoned for messenger. Signal looks like a winner though.