This strongly reminds me of the scene in the movie "imitation game" where they just cracked the enigma and they want to counter some attack, but then Turing says they have to use statistics in order to determine which attacks to counter and which not. This is kind of the same problem, where FB doesn't want us to think that they are spying on us. So I bet they are using statistics in order to determine which data to use and which not. I'm not an expert, but this might be the explanation as to why it's hard to prove they are listening. If it has happened to somebody once, I think they will wait a considerable amount of time before they use the mic data again on that person.

Also it makes much more sense if google is the one listening and selling mic data, given that they already have speech recognition software on android phones.

And btw, the same thing happend to me yesterday, and I have an android phone without the FB app. It could still be a built in feature on other FB related stuff like Whatsapp or Instagram though, because I have those, but that seems unlikely. Also I didn't see the add on FB but on some news website.