That Product You Looked at Once Follows You Everywhere
After looking at shoes on Amazon, you open a news site and there's an ad for those same shoes. Open Instagram - same shoes again. YouTube too. It feels like the entire internet is stalking you.
This is an advertising technique called "retargeting" (or "remarketing"). It's not a coincidence - they're intentionally following you.
How Retargeting Works
Retargeting ads operate through the following steps.
- You view a shoe product page on Amazon
- An ad tag (a small program) embedded in Amazon's site saves a cookie in your browser
- You visit a different site (news site, social media)
- The ad slot on that site reads the cookie and recognizes "this person was looking at shoes"
- The ad network displays a shoe ad to you
This entire process happens in a split second (tens of milliseconds) before the page finishes loading. Every time an ad slot appears, a real-time auction (RTB: Real-Time Bidding) takes place, and the ad from the highest bidder is displayed.
Why Ads Keep Following You Even After You've Bought the Item
You may still see ads for the same shoes even after purchasing them. This is a weakness of retargeting.
- The ad system records that you "viewed the product page" but may not accurately track that you "made a purchase"
- The ad campaign settings don't have "exclude users who already purchased" configured
- Tracking continues until the cookie expires (typically 30-90 days)
The frustration of "I already bought it!" is also wasted ad spend for the advertiser. Well-managed ad campaigns exclude purchasers and switch to related products (recommending socks to someone who bought shoes).
How to Reduce the Ads Following You
- Regularly delete cookies: Deleting third-party cookies from your browser settings resets the tracking
- Ad tracking protection: Enable your browser's "tracking prevention" feature. Safari blocks third-party cookies by default
- Turn off ad personalization: You can disable personalized ads on Google's "Ad Settings" page
- Ad blockers: Block ads entirely with extensions like uBlock Origin
- Incognito mode: Since cookies aren't saved, it prevents retargeting tracking (though you're still tracked until you close the window)
The End of Third-Party Cookies
Third-party cookies, the foundation of retargeting ads, are heading toward deprecation from a privacy perspective.
- Safari: Completely blocks third-party cookies since 2020
- Firefox: Blocks by default since 2023
- Chrome: Was considering deprecation but reversed course in 2024, moving toward giving users a choice
Without third-party cookies, current retargeting ads won't function. The advertising industry is exploring alternative tracking technologies to replace cookies, such as browser fingerprinting and Google's Privacy Sandbox.
Summary
The same ads following you everywhere are the result of cookie-based retargeting. A real-time auction occurs every time an ad slot appears, displaying ads based on your browsing history. You can reduce tracking by deleting cookies and enabling tracking prevention features. To see what tracking signals your browser currently exposes, try checking your IP address and browser fingerprint on IP確認さん.