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I do not have the facebook app on my phone. I downloaded Instagram and it magically knew one of my email addresses (not the one used with my facebook account). I deleted it, mashed the keyboard for the new email address, declined syncing contacts, blocked the permission as well as every other permission, and lo and behold it still shows all of my facebook friends on instagram as suggestions. Fucking creepy.

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Even if Facebook doesn't get the data through you, it still gets your data from people you know.

It was people who don't have my phone number or any other information including people who I am not friends with and only messaged once from a buy/sell group.

I am guessing because I used the app at some point on my phone it fingerprinted it, then Instagram goes and fingerprints it when you install and links your account even when you decline to do so.

Spammers get your email address the same way by hackers who hack your friends.

Lots of ways this could happen but people often forget about GPS/location services which are very accurate. I'm not sure if you have never had FB app on your phone, but even if you never installed FB app on your phone there could still be geo-associations inferred.

Think about the times your phone is near your friends phones, how your phone probably sits in the same space every night, the overlap between your IP location to whatever other devices connect behind your NAT which also send data to FB.

If you previously had Facebook or Messenger installed on the phone it's possible they saved some user info on the device or in the cloud linked to your device ID.

AFAIK device uuid has been unique to device+app for at least 2 years now (iOS).

device + app having a unique UUID has been going on longer than that.

3 years ago uber got popped for this:

https://www.theverge.com/2017/4/23/15399438/apple-uber-app-s...

I can tell you that fingerprinting is still possible today.

It’s company not app on iOS, you get a new one if you delete every app from one publisher

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