Pretty much no-one uses any of those for personal stuff, Slack does have some open-source project stuff in there.
FB Messenger seems to be highly siloed, I never use it and I know only one person who uses it as their main communication platform.
Whatsapp has reached critical mass in many countries outside of the US. People use it because everyone uses it - not because it's especially good in any way.
Signal, Matrix etc are for idealists, who have a future vision of what the platform will develop into in mind.
My personal favourite is Telegram. It has a good mix of bots, API, native clients and channel management tools.