How Facebook Figures Out Everyone You’ve Ever Met

Excerpt from this article:

In the months I’ve been writing about PYMK, as Facebook calls it, I’ve heard more than a hundred bewildering anecdotes:

A man who years ago donated sperm to a couple, secretly, so they could have a child—only to have Facebook recommend the child as a person he should know. He still knows the couple but is not friends with them on Facebook.

A social worker whose client called her by her nickname on their second visit, because she’d shown up in his People You May Know, despite their not having exchanged contact information.

A woman whose father left her family when she was six years old—and saw his then-mistress suggested to her as a Facebook friend 40 years later.

An attorney who wrote: “I deleted Facebook after it recommended as PYMK a man who was defense counsel on one of my cases. We had only communicated through my work email, which is not connected to my Facebook, which convinced me Facebook was scanning my work email.”

Connections like these seem inexplicable if you assume Facebook only knows what you’ve told it about yourself. They’re less mysterious if you know about the other file Facebook keeps on you—one that you can’t see or control.