By the way I think it's possible to leave facebook while keeping messenger. At this point, I feel like most of the people I know who are on fb use it or messenger rather than anything else.
If you are deciding to "leave Facebook" as protest to the fact that:
Facebook has played a significant role in undermining democratic values around the world
then that means you need to leave Messenger, Instagram, Whatsapp, tbh, Facebook (product), Oculus and every other Facebook owned property.
I don't see how people aren't making this connection.
If you want to leave FB on moral principals, then dropping all of their products is the only thing that would be impactful. Otherwise you're just shifting where you give them revenue.
Signal and iMessage are my favorites personally.
SMS doesn't require an app that requests invasive access to information.
Doesn't mean that won't change, but their current policy states it isn't happening now.
Secret messages can't be harvested, but aren't portable between devices.