I highly doubt that they actually managed to do this, at least any time recently.

As another commenter noted, Android alerts you when an app is accessing the microphone in the background, and it would also absolutely destroy the phones battery life more than the FB app currently does. The only way that we have the “Hey Google/Siri” command prompts active all the time is with custom hardware not available to the apps, and certainly not without Android knowing about it.

Maybe they actively listen while the app is open, but even then I think recent Android/iOS would let you know about that.

That is not the same thing as listening in the background.