Ask Slashdot: How Much of Your Online Browsing Can Advertisers See?

dryriver writes: We all know the phenomenon of browsing from an internet site A to a completely unrelated internet site B, and having identical ads follow you from site A to site B. Logic suggests that some kind of advertising system is following you from site A to B, and possibly onto subsequent sites C, D and E as well. Logic also suggests that this advertising system can now put together a nice long list of whatever you are looking at online. So here's the question: How much of your online browsing is "monitored" or "logged" this way by advertisers? Can there be any realistic expectation of privacy on the internet if the default behavior of advertisers is to track you as much as they can?

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[turkeydance]

of it

They use tracking IDs.

[Static]

Advertising content puts tracking cookies in your browser. Due to how cookies work, they are associated with the advertiser, not the website you're looking at. This means that the advertiser will see the *same* tracking ID whenever their content appears regardless of the site they're advertising on. Since they know what sites they're advertising on, they can match that with the tracking ID they've dropped on you to assemble a history of what sites you're browsing through. Including giving you the same ads.

This is the "forgotten" reason why people run ad-blockers: to nix the tracking data across websites!