ads in the way youre describing are often based on local wifi association and not microphone snooping.
Your friend recently has searched for "the hottest restaurant town." Your friend joins your wifi.
Now that your friend is on your wifi, Facebook assumes you and your friend have lots in common. Facebook then assumes that YOU also would probably like to know more about the hottest restaurant in town, and so it shows you ads for it.
Unless you both are very careful to activate your vpns, FB will figure you out.