I can't see a single thing WhatsApp or Facebook messenger can do for me that signal can't do better because I simply don't need SoMe on my phone. I need encrypted SMS. Anything more than that is worse than what I already got.
To paraphrase someone young from my family: "Facebook is for old people". So is WhatsApp and Facebook messenger. But of course it's just an anecdote that covers 100% of the people I happen to know. So from my bubble I don't believe for a second that they have a future. Signal might. As encrypted SMS but not otherwise.