> How can it be that having intimate knowledge of someone would not allow you to sell them more stuff? John has a 10th wedding anniversary coming up. Mary is single and goes to the gym every day. Steve just got a promotion and likes the BMW 3 series. Michael is overweight but has just gone on a diet. Are such details of no significance determining what ads to show these people?
It's possible that some combination of efficient markets, semi-inaccurate/incomplete tracking data and chaos theory combines to make it mostly irrelevant.
So Steve just got a promotion and likes the BMW 3 series, but advertising to him is useless because he has already decided to buy a BMW 3 series. Whereas Larry has had the same job for ten years, and the same Ford for ten years, but if you put a luxury car ad in front of him it may get him to take a test drive and actually create a new customer.
The data says Mary is single and goes to the gym every day, but it only thinks she's "single" because she's in a committed long-distance relationship and isn't interested in dating anyone else. And she's a fitness expert who is willing to spend her time researching fitness products, so she already knows everything there is to know about those products, already buys the ones she wants, and advertising them to her isn't going to create any new exposure. Whereas Jane never goes to the gym, so you might actually sell her a gym membership or a piece of fitness equipment because she doesn't already have one or know anything about it yet.
John has a 10th wedding anniversary coming up, but he has also known exactly what he's going to do for years. Whereas he has a third cousin whose wedding is coming up and has no idea what to get, so you should be showing him ads for toasters and flatware rather than jewelry and chocolate.
In general, you may do better to advertise your stuff to the people who aren't already interested in and knowledgeable about it. Which actually looks kind of a lot like random scattershot rather than targeting.