You don’t need cookies to track conversions, why? Because the conversions you were tracking with third party cookies were not real anyway. Let me count the ways.
Individual ads do not cause individual conversions
Advertisers have been misled by data to think that individual ads caused individual conversions. But, as a human, when was the last time you bought something just because you saw one display ad, one video ad, or one search ad? Seriously, think about this for a minute. Even if you did purchase something once or twice immediately after seeing an ad, are you a human who can be repeatedly convinced to buy something simply because you saw a single ad? I hope not.
Obviously, ads play a role in the awareness of a previously unknown product, or as a reminder to buy a product or brand you already know. Modern consumers are too smart to be tricked into buying something after seeing just one ad (I hope). Modern consumers are also armed with more information at their fingertips than ever before. Even if they wanted to buy something after seeing an ad, they are likely to search for more information about the product before making the purchase, especially if it were a more expensive item. They read Amazon reviews; they see what folks are saying about the product on Instagram or YouTube. All of that information further informs their purchase, not just seeing one ad.
So the whole notion of using cookies to track which individuals were exposed to an ad and which individuals went on to buy something is fundamentally flawed. It is just as flawed as the notion of “the right ad to the right person at the right time” being good digital marketing. The availability of third party cookies led advertisers down the 10-year-long rabbit hole of collecting more and more information about users, using that information to target users with ads that the advertiser thought were relevant, and then using the cookies to attribute certain sales to certain “exposed” users — i.e. trying to do attribution down to the cookie level (which users converted because they saw an ad).
If you understand that human consumers don’t buy something because they saw one ad, you will also understand that…
