Well, at my last employer we had our own systems analyzing traffic, we had integrations with third parties (DoubleVerify being the biggest) to detect fraud, our clients often had their own third party integrations to detect fraud and would complain if we hadn't found it but they did at too high a rate, etc.

Honestly after the ad images/video, all the fraud detection javascript from the advertiser, the publisher, the advertiser's system of choice, the publisher's system of choice all doing "trust but verify" with each other is the largest contributor to how big ads are.

Every been to a big break and motor store and they asked for your zip code? That is because they have someone in the back office to wants to see if the ads - dead tree paper - they spent to your zip code worked. It isn't hard to work the numbers from there, but it is tedious and requires some statistics.

Heh, I always find it funny when I buy a non-recurring thing, but keep seeing ads for it.

Best was when I kept seeing ads for a thing I was selling. I visited some big box sites about it to copy the specs on it.

The vacuum problem... the issue is that it's kinda hit or miss to come up with items that are related to the vacuum and to do it over a time period.

I think if ads were smarter, you'd get ads for your vacuum's filter a few months after buying it and maybe replacement pieces just after warranty, but what do I know?

Maybe some Toyota ads after owning a Mercedes for >5 years?

No actually. Post codes are a recent thing here, and are mostly not hierachical. Also the standard reaction would be wondering why they need any info to process an instore purchase.

ZIP code is also used for credit card verification.

Only at gas pumps.

And that strategy still doesn't account for word of mouth.

actually it does. That is why you need a full time statistician on staff (likely a department), they spend a lot of effort finding and accounting for confounding factors.

Amazing that we have an industry whose biggest component is keeping out fraud in the industry. Imagine pitching that to a VC.