To deactivate Facebook Messenger, go to the Messenger app > hamburger menu > cogwheel icon > Accounts Center > Personal details > Account ownership and control > Deactivation or deletion. If you don't ...
But Facebook does not delete the account. Why? So you'll be able to reactivate it later. It says so as you deactivate: "This can be temporary." And you can still use Facebook Messenger. Just in case ...
or you can even just delete your Facebook profile while keeping your account to use for the separate Messenger app. If you want to reactivate your account afterwards, you simply need to log back into ...