If you’re a Facebook user, you’ve probably experienced that feeling of shock when a long-forgotten friend or ex pops up in the site’s “People You May Know” section. This week, The Washington Post took a deep look into how the site can know so much about your relationships.

The paper’s article mentioned a Reddit user who recently (and finally) joined Facebook. Upon signing up, the user discovered that the site already knew all of his friends, and he was understandably unsettled. Facebook has long been questioned for this practice; theories ranging from phone contact scraping to reverse-image searches have been floated for the site’s seeming all-knowing ability to comb through your past. However, as found by the Post, the answer is actually much simpler.

When you register for Facebook, the site asks permission to scan your email contacts or mobile contacts, depending on what device you’re using to access the site. It then finds which of your friends are already on the site and combines that knowledge with other information you provide, including where you live, where you were born and where you went to school. It then uses each one of your social interactions on the site to build a more complete picture of your network. From there, it isn’t hard for the site to guess people you may know.

While Facebook’s algorithms can only do so much to guess your social circles, the site is dedicated to constantly improving its models. In other words, don’t expect Facebook to know less about you any time soon.