With facebook web it's pretty easy to say as you have to give permission via your browser. Otherwise, for Chrome, you'd have to assume that Google works with Facebook to help them make more money. Rather unlikely.

How did you ensure that no other apps were listening in the background?

I don't understand--how would other apps trigger related facebook ads?

Send some data to an ad network or something that feeds it, Facebook ads somehow pick that up. Not beyond the realm of feasibility. Try using a just-factory-reset phone with JUST the Facebook app installed. There is science to be done here.