Elena Scotti/FUSION

Facebook's ability to figure out who you know in real life can sometimes be unnerving. The "People You May Know" list can be uncanny, leading many users to ask, "How do they know I know that person!?"

Well, it's possible that you actually told Facebook yourself. If at some point in the past you uploaded your email or phone contacts to the social network, it can link them to Facebook accounts and figure out you know someone.

You may not remember doing this. You may be convinced you'd never do that. But you may be wrong.

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Thankfully, there's a way to check. You can see email contacts you've uploaded to Facebook here and both email and phone contacts you've uploaded via Facebook Messenger here.

I discovered 7 random email addresses that I'd uploaded in 2009. One of my colleagues discovered he had over 200 email addresses and phone numbers that were taken from his phone. He doesn't recall having chosen to upload them, but must have clicked a button at some point handing them over.

His list was huge

As Facebook explains: "Only you will be able to see your contacts and info about them, but Facebook will use the info you've uploaded about your contacts to make friend suggestions for you and others and to help us provide a better service for everyone."

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Note the "and others," which may support my hypothesis that a psychiatrist's patients were recommended to one another as friends because they all have her number in their phones.

If you're not happy about Facebook knowing all these contacts, there's a handy "Delete All" button.

If you don't want your contacts on Facebook's servers, use this button.

And you probably want to go to the settings on your Facebook Messenger app if it's on your phone. Go to "Me" and then "People" and turn off Contact Syncing.

Turn off sync if you don't want Facebook to know every time you add a new contact to your phone.

If your list is completely empty, congrats! You're better at data hygiene than most. However, you're not in the clear. If anyone has uploaded your email address or phone number to their own list, then Facebook knows you know them.

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As one Twitter user eloquently put it, "in a two-way relationship, Facebook only needs one person to give up the goods."


Here are the two sites to check out:

https://www.facebook.com/mobile/messenger/contacts/

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https://www.facebook.com/invite_history.php