Facebook may only need to recognize a few keywords. The one that companies had payed for. It wouldn't be that many. An approximate recognition is good enough. So processing and data transmission could be small.

As long as there is no uncontestable proof, the doubt must benefit Facebook.

The testimonies are not uncontestbale proofs. Considering the million people uisng the facebook app, there will always be a small set of people who will see advertisement of what they just talked about. That is statistic. Some of these, understandably chocked, may be very vocal about it.

We also know that people are payed to spread fake news and manipulate people. So I'm cautious. The truth is unknown, and the longer it takes to get uncontestable proofs, the less credible the listening thesis becomes.

Facebook can't do much appart to let it rain. Open source is the way I would go. That is one way to calm down paranoid or conspirationist people.