I worked at Facebook for four years (three of which on the iOS app) and it's implausible to me that they're using the microphone to listen into people’s conversations, at least on the flagship apps (Messenger, Facebook, Instagram).

They'd have to be using some very sophisticated techniques to hide internally that they're doing it, given that engineers can all see the entire codebase. I was pretty intimately familiar with every bit of the build process; it'd have been hard to miss some injection of secret code. Also, the benefit would be minuscule, and the threat to their reputation if caught (given that Zuckerberg has very explicitly claimed not to be doing this) would be huge.

I am almost certain that it's not happening on Android either, but I'm not as familiar with that codebase, so I won't claim to know first hand.