Facebook has a feature that allows you to tag friends in photos. They encourage you to click on the individual in the photo and then “tag” their user name.
This helps Facebook build an image library from multiple angles and perspectives. This data is processed by Facebook to create their automatic facial recognition software. Facebook claims it can accurately identify people’s faces in photos – literally among billions of photos in the world and that it can do this in seconds. They argue that this is of value to you as it alerts you when your friends share a photo of you. In fact, it enables Facebook to track your real world, offline activities.
This database can and likely is used by third parties, such as national “security” agencies, to track members of the public as they move about.