I have a suspicion it’s not Facebook or Google listening in, but rather other third party apps. In fact it’s not even the third party apps but the libraries/frameworks they use to show ads.
Unless we're talking about an electret capsule with a physical LED wired into the supplying power rail that is switched off when the mic is not in use, you have to trust software.
And good luck with that after the patriot act. I am not implying the NSA has a microphone backdoor, but if they had and someone abused it, how would you know about it?
Listening in for keywords and only send text/audio when keywords are spoken isn't only good for ads, that would be dream of any intelligence agency. And since Snowden have been a few years.
"Citizen four" by Edward Snowden.
Those chips do not log or transmit the audio they have access to.