https://www.theguardian.com/technology/2017/oct/30/facebook-denies-eavesdropping-on-conversations-to-target-ads-again

Social network’s head of ads dismisses rumours that Facebook app uses smartphone mics to listen in, serving ads against what it hears

Facebook has again denied eavesdropping on conversations to target adverts, following persistent speculation on the part of users who have received suspiciously timed promotional messages.