How Facebook Figures Out Everyone You’ve Ever Met

# # #

In case you wondered how Facebook knows who to recommend on their People You May Know feature: 

Behind the Facebook profile you’ve built for yourself is another one, a shadow profile, built from the inboxes and smartphones of other Facebook users. Contact information you’ve never given the network gets associated with your account, making it easier for Facebook to more completely map your social connections.

Source: How Facebook Figures Out Everyone You’ve Ever Met