Facebook has repeatedly denied eavesdropping on conversations through the phone's microphone.
"Some recent articles have suggested that we must be listening to people’s conversations in order to show them relevant ads. This is not true. We show ads based on people’s interests and other profile information – not what you’re talking out loud about."
Turn Facebook's microphone access off
In iOS, go to Settings > Privacy > Microphone select off.
On Android, go to Settings -> Personal > Privacy and Safety > App permissions > Microphone Facebook off.