How many photos do you need to tell someone is socially connected? When combined with time and location data, the number could be as low as three. And that finding may not be tied to more than just photos (think credit card transactions, public transit tickets, and cell phone records).
“While it’s obvious that a photo you post online reveals information about what is pictured in the photo, what is less obvious is that as you post multiple photos you are probably revealing information which may not be pictured anywhere.”
An model that can predict these connections could lead to highly targeted social advertisements, without requiring you to provide Facebook/Twitter credentials.