Facebook really is spying on you, just not through your phone’s mic

The accuracy of Facebook's ad targeting sometimes leads users to believe that Facebook is spying on them by tapping the microphones in their phones. Facebook has denied the practice - and is likely telling the truth because uploading and scanning the amount of audio data such a system would involve an unattainable amount of processing power to understand context. 
It sounds believable: Joanna Stern's mother told her to buy the decongestant Sudafed in the morning, and by afternoon she sees an ad for it on Facebook. Instead, Facebook gleans its insights from the data it collects by watching us move around the web and, in some cases, in the physical world. The Sudafed ad was generated like this: earlier in the day Stern bought tissues and the nasal spray Afrin at Walgreens, keying in her phone number in order to get loyalty points. A third-party data collector added the contents of her shopping cart to the purchase history it acquires from Walgreens, and then sold it to Sudafed manufacturer Johnson & Johnson, whose specified targeting coupled with Facebook's account data, led to an ad targeted at Stern.
There are ways to opt out of a fair amount of data gathering, but it requires discipline: don't use loyalty cards, or register them to a phone number or email address you don't use for other things; opt out at the data brokers' websites; turn off location tracking in your phone; and check on the ads shown by other apps on your phone. Finally, use ad blockers and opt out of targeted ads on Facebook itself.

https://www.wsj.com/amp/articles/facebook-really-is-spying-on-you-just-not-through-your-phones-mic-1520448644?__twitter_impression=true
Writer: Jessica Stern
Publication: Wall Street Journal