That's terrible. I hope that removing all of Facebook's software from a phone and using cash prevents that. I hope you're joking about the audio signal.
Not quite used for in-store advertising, but Google's "Nearby Messages API" does use "near-ultrasound" audio signals to communicate with other devices: https://developers.google.com/nearby/messages/overview. I don't know if it would work at a larger scale, though (maybe you couldn't allow dogs in your store!).
