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Ask HN: How does Facebook know what I buy on Amazon or vice versa?

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Ask HN: How does Facebook know what I buy on Amazon or vice versa?

#1

Post by kmonad »

Following just happened: An hour before lunch I googled and visited websites that sell bicycles. I also visited Amazon during this research. I then bought a bike from one of the manufacturers' websites. A few hours later I browse facebook and see ads to this manufacturers' bikes in my newsfeed, via an Amazon sponsored ad.

I use one browser (Safari) for facebook exclusively, and browsed the bikes / Amazon / made the purchase on Chrome. I have different email addresses for facebook, amazon and, well, google.

Re: Ask HN: How does Facebook know what I buy on Amazon or vice versa?

#2

Post by trjordan »

Some data matching isn't done with straight cookies. You visited from the same computer, same IP. It may be a guess done by comparing IP + installed fonts.

The other possibility is that your multiple email addresses have been matched as the same person. So even though you use different browsers and have different cookies, they're collapsed on Facebook's side.

Re: Ask HN: How does Facebook know what I buy on Amazon or vice versa?

#3

Post by jdavis703 »

These companies can also track you by IP address. I've had conversations with co-workers about them considering a specific large purchases, and then seen ads for what we talked about popping up. I'm assuming that if the CPM is low enough, many ad-retargeters will take the risk of targeting ads based on IP address alone.

Re: Ask HN: How does Facebook know what I buy on Amazon or vice versa?

#8

Post by aresant »

Straightforward:

1) Amazon tracked your research and likes to use " ad retargeting" if they see you leave before checking out to remind you to come back and purchase.

3) FB offers advertisers a variety of retargeting means + insanely advanced cross-device tracking. Like a building full of PhD's advanced. Maintaining privacy by logging in from different devices / accounts / etc is a thing of the past if you EVER cross-pollute between browsers / devices / etc the signal is picked up and then compared with browsing behavior etc to get a strong profile. (1)

(1) https://www.facebook.com/business/a/performance-marketing-st...

Re: Ask HN: How does Facebook know what I buy on Amazon or vice versa?

#9

Post by TekMol »

I have had similar experiences. Others have already mentioned some cross-browser fingerprintig techniques. One of the worst that many people don't know about is that browsers hand over your local IP. Check this proof of concept:

http://net.ipcalf.com/

The media device IDs the browsers provide look even worse:

https://jsfiddle.net/u4n4s296/

I am not sure if these are unique to the device type (for example a certain soundcard model) or to the device itself. If it's the latter, then that is an indestructible cross-browser cookie right there. EDIT: As per icebraining's comment, in Firefox they are not not cross-domain, not cross-browser and get randomized when you delete your cookies.

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