Northeastern Universtiy year long test of Android phones.
For one full year, they monitored a set of Android phones testing 17,000 apps, including Facebook and over 8,000 apps that sent information to Facebook. This was to see if any of them used the microphone to collect what you were talking about and used that to send out the conversations.
The conclusion is that none of the apps send out audio data.
However, they did find that instead of sending out audio data, they do send out screen shots. Over 9,000 apps of the 17,000 had permission to access the camera. One of the big issues is that these apps used a service call Appsee to take screen shots and videos of your screen activity.
The best thing for a user to do is to review the apps on their phone and see what permission the apps have. And when you look at the app you need to decide if it really needs access to your camera or microphone. If it does not then deny the app access.