Gizmodo has an interesting blogpost about Facebook called "Facebook Figured Out My Family Secrets, And It Won't Tell Me How." The author says:
"Rebecca Porter and I were strangers, as far as I knew. Facebook , however, thought we might be connected. Her name popped up this summer on my list of “People You May Know,” the social network’s roster of potential new online friends for me. The People You May Know feature is notorious for its uncanny ability to recognize who you associate with in real life. It has mystified and disconcerted Facebook users by showing them an old boss, a one-night-stand, or someone they just ran into on the street.File under creepy.
What makes the results so unsettling is the range of data sources—location information, activity on other apps, facial recognition on photographs—that Facebook has at its disposal to cross-check its users against one another, in the hopes of keeping them more deeply attached to the site. People generally are aware that Facebook is keeping tabs on who they are and how they use the network, but the depth and persistence of that monitoring is hard to grasp. . . . Rebecca Porter, we discovered, is my great aunt, by marriage. She is married to my biological grandfather’s brother; she met him 35 years ago, the year after I was born. Facebook knew my family tree better than I did."