Some people still believe that their phones are listening in to gather data that will inform targeted advertising or compromise their privacy. Facebook has directly denied that its apps are listening in Congressional hearings, but there hasn't been a rigorous scientific study of the issue. Academics at Northeastern University, however, have finally done just that (though only on Android devices).
As reported by Gizmodo, researchers Elleen Pan, Jingjing Ren, Martina Lindorfer, Christo Wilson, and David Choffnes studied more than 17,000 apps, including ones from Facebook and those that send information to Facebook, to see if any of them were capturing audio via Android phone microphones. As you might have guessed, the researchers found no evidence of apps activating the microphone or sending audio on the sly.