I had forgotten (or never realised) that ‘people you may know’ was originally a LinkedIn feature before Facebook poached it. This article covers how the shady world of shadow profiles enabled Facebook to turn this cute idea into something spooky.
If Facebook sees an email address or a phone number for you in someone else’s address book, it will attach it to your account as “shadow” contact information that you can’t see or access.
That means Facebook knows your work email address, even if you never provided it to Facebook, and can recommend you friend people you’ve corresponded with from that address. It means when you sign up for Facebook for the very first time, it knows right away “who all your friends are.” And it means that exchanging phone numbers with someone, say at an Alcoholics Anonymous meeting, will result in your not being anonymous for long.