I doubt fb is involved these fears but it’s funny to imagine that they seeded those rumors to distract from what they are really collecting.

It mostly shows that there is no shortage of rumor generators and listeners

It's pretty hard to prove a negative. How do you go about doing it to your own satisfaction? And what would be sufficient proof to you that it happens?

If it was ever found out that Facebook was eavesdropping on a large portion of internet users via their phone microphones to target ads, the consequences to Facebook would be extreme and far reaching. It’s simply not worth the risk to target ads slightly better.