Considering the original claim was "zero indication that the mic is hot" and now it's "zero indication that the mic is hot if the screen is off", I'd say that the goal post has moved considerably.
But if you want to know if Facebook is listening to you through the iPhone microphone, you should probably look at the screen for the indicator. iOS apps can't start recording on their own in the background, there's no API for that. If they are listening to you, they'd have to start the audio session in the foreground, which would allow you to see the indicator.
https://developer.apple.com/forums/thread/65604
https://stackoverflow.com/questions/70562929/how-to-start-au...
(Unless you believe that Facebook is using some kind of a private system API for this and is passing through the App Store checks)