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Facebook's "People You May Know" feature pops up occasionally to get you befriending people you might know tangentially through your different networks, expanding the ever-reaching tentacles of information Facebook has on you. And if you have location services turned on, it's using your location to match you with potential friends.
The company uses the GPS on your mobile device as a signal for whether you may know a fellow Facebook user, Fusion reported. "People You May Know" shows up on your home page, your friend's pages, and ad space on the site, following you around with names and faces of the people you could potentially be connected with.
The company does not explicitly disclose that location data is being used to match friends. From its Help page:
People You May Know are people on Facebook that you might know. We show you people based on mutual friends, work and education information, networks you’re part of, contacts you’ve imported and many other factors.
Facebook's location services might be turned on for a number of reasons—checking in to venues, tagging yourself and friends at airports, looking for local events, and even finding nearby friends (if you want to get super invasive). While all of these use cases are understandable, if not a bit creepy, using location to surface potential friends could be a massive violation of privacy.
Fusion reports the story of an anonymous man who attended a get-together for suicidal teens and later saw a fellow parent from the gathering as a suggested friend on Facebook. They had not exchanged contact information—their intimate location was a factor Facebook used to suggest they befriend one another.
Facebook doesn't rely exclusively on location data to show you friend suggestions, but it is one factor that, if paired with another, may signal to Facebook you know someone. And those networks can be huge.
Let's say you belong to a network of people who went to Arizona State University, my alma mater with more than 80,000 students currently enrolled. That's a huge number of people, from just one enrollment period, that could belong to the same network. If I cross paths with another ASU alum, Facebook could potentially assume I know them, even if we graduated years apart and never met.
Facebook considers this a feature, not a bug, but it could be invading your privacy. For instance, although Facebook requires people use their "real names" on the site, many people don't—and sometimes people use pseudonyms in real life, too. So if Facebook matches a person to someone via location and other data, and the person recognizes the profile photo with a different name, Facebook just gave away personal information that person didn't want exposed to the internet.
If you don't want Facebook to use your location data as friend suggestions, you can turn it off in your device's settings. Open up Settings and tap Location Services. In Location Services, you will see a list of apps that use your location, and you can turn them off individually.