[SmellyGeekBoy]

> I'm not a programmer so I don't know, but: Wouldn't it be trivial for someone who knows how to write Android software to monitor if an an application is accessing the audio input device?

Or even by MITMing the connection and looking at the network packets. But yes, it's not outside the realms of possibility to hook into the microphone driver on a rooted Android phone and check when it's being activated.

I'm not one to leap to Facebook's defence and if this is happening it needs to be shut down ASAP, but I suspect that there would be at least some credible evidence out there if it were indeed the case.