Yes, tracking last-touch is easy.  but last touch is far from the whole story, and once you move away from simply tracking last touch, it gets really complex.<p>Sure, the user saw the facebook ad or clicked on the facebook ad.  But... does that just mean that facebook is good at showing ads to the sort of people who already buy those products?<p>(I know that when I&#x27;m running without ad-blocker, I already own most of the products that correctly-targeted ads try to advertise to me.  My argument is that the ads are merely following my preferences, not driving those preferences.)