This has been tested and proven time and time again. Google and Apple give apps a method to access the mic that the OS can allow or deny. This is what the apps are supposed to use.

Facebook doesn’t use that method for passive monitoring, only for active engagement. Don’t looks like it’s working when you go the use the thing and it’s disabled but it’s still listening to you.

If you must use such a social network, never ever ever use the mobile app. Use the website or don’t use the service at all.