Having worked in ad-tech for a while, and dealt with cookie matchers and retargeting -- I'm actually surprised to the degree to which companies keep pursuing this stuff .. because it wasn't clear to me from the statistics that it was actually effective.

I was involved in the relatively early days of this, so probably techniques have improved. But I worked for a startup that was doing search retargeting, among other things, and when I ran the numbers I just didn't see a huge increase in click-through rates. The data just wasn't there.

Like I said, I'm sure techniques have improved. But I still continually see ads retargeted to me that are both creepy and ineffective. Marketing me things I've already bought. Or lost interest in. I can count on one hand the number of times I've seen an ad based on retargeting that I thought "oh yeah I should pursue that."