Try this.... lets say you use facebook, you have the app on your phone, Now you strike up a conversation with your other half or some random person about how you need laundry soap or some other specific common item. give it a couple hours or some short amount of time. open the face space app and start scrolling in you news feed.... BAM they start showing you ads for that specific thing you want or was needing.
it also works in a way that you use your internet browser and "google" as a search engine. search google for some common specific item, on your phone or PC. then later open face space app and start scrolling... BAM there it is again....
trust me, it's happened to me multiple times.