They are probably not listenting in on your conversations, they just have about 5,000 data points on every single one of their users.
There's a good reason so many Americans believe that Facebook has been listening in on them. The ads Facebook/Instagram serve are so personalized it almost feels like we're being watched and listened to.
The reason Facebook is able to show you ads that you may have been talking about recently is because they know more about you than anyone. They follow you to almost every page you visit on the web, not just Facebook and Facebook owned sites. They analyze everything you like and everything your friends/followers like.
The belief that Facebook listens in on conversations isn't just limited to a few Americans. It has become such a widespread belief that Congress has questioned Zuck himself about the theory.
If you do believe Facebook is listening in on you, you can always disable their access to your microphone...
-Then tap Privacy.
-Select Facebook, and disable access.
While they won't have access to your microphone anymore, you'll still likely be served eerily relevant ads on a daily basis. Remember this company and their database knows more about you than anyone else in the world and their data on you grows every day.