I recently found out about this. Facebook can figure out people you might know by analyzing the pattern of dust and scratches on a camera lenses. Facebook filed a patent for their “people you might know” feature, the thing that recommends someone that you might want to become friends with on the social network site.
And the way that it works is this: Imagine we’re at an event and I’m taking pictures. I take a photo of you that you really like. We don’t know each other but I show it to you, I send it to you and you post it on your Facebook page. I do the same thing with someone else.
So none of us are in the pictures, none of us know each other, I’ve just taken a picture of you and someone else, and you both posted them on your Facebook pages.
Facebook can then analyze the scratches and dusts on your camera lenses that were on my camera and find the invisible artifacts that those scratches left in the photo to determine that you both had your pictures taken by me at the same time and then recommend that you become friends.
When reporters found this patent, Facebook denied that they were using it in their system but there’s no way to know if they’ve started using it since or if they’ve put it in something like Instagram and WhatsApp.
This is how much trouble we have to protect our privacy.