Kashmir Hill:

In real life, in the natural course of conversation, it is not uncommon to talk about a person you may know. You meet someone and say, “I’m from Sarasota,” and they say, “Oh, I have a grandparent in Sarasota,” and they tell you where they live and their name, and you may or may not recognize them.
 You might assume Facebook’s friend recommendations would work the same way: You tell the social network who you are, and it tells you who you might know in the online world. But Facebook’s machinery operates on a scale far beyond normal human interactions. And the results of its People You May Know algorithm are anything but obvious. In the months I’ve been writing about PYMK, as Facebook calls it, I’ve heard more than a hundred bewildering anecdotes: