[patcheudor]

This is the correct answer. We noticed this happening in our small lab and decided to run some tests and in fact if one person searched for something odd like 'Swedish fish' then visited sites selling Swedish fish, everyone in the lab would start to get ads for candy and snack food. Similarly, searching for "mountain bike" would result in everyone else seeing ads for bikes and bike accessories. It sort of became a game to figure out what the test search term of the day was.

[otakucode]

Many would probably hear of experiences like that and think 'hey that sounds unintentional or like behavior the advertiser wouldn't want' but it actually sounds brilliant from a pure effectiveness point of view. You don't just show an ad to an interested person.... you increase the chance that the person will hear about the thing from people in their immediate vicinity. And what's the best advertising in the universe that no one can buy? Word of mouth.

Advertisers had better be paying through the nose for this sort of access, because it's insanely valuable.

[jackgolding]

We get this all the time in our office for salesforce (half of our team uses it.)