Right, like the example from Target. It is possible that facebook has access to a stream of Target purchases or credit card purchase information. I'd be 0% surprised if there was a data sharing agreement with credit rating companies.
Even if they just knew the price of the purchase, combined with the job (burn risk) and location data (traveled from work to nearest store with pharmacy during the day), it's possible that you might be able to infer a burn with enough accuracy to be valuable to advertisers.
Even if they just knew the price of the purchase, combined with the job (burn risk) and location data (traveled from work to nearest store with pharmacy during the day), it's possible that you might be able to infer a burn with enough accuracy to be valuable to advertisers.