One big difference between faces and other biometric data, such as fingerprints, is that they work at a distance.
Anyone with a phone can take a picture for facial-recognition programs to use.
FindFace, an app in Russia, compares snaps of strangers with pictures on VKontakte, a social network, and can identify people with a 70% accuracy rate.
Facebook's bank of facial images cannot be scraped by others, but the Silicon Valley giant could obtain pictures of visitors to a car showroom, say, and later use facial recognition to serve them ads for cars.
Even if private firms are unable to join the dots between images and identity, the state often can.
China's government keeps a record of its citizens' faces; photographs of half of America's adult population are stored in databases that can be used by the FBI.
Law-enforcement agencies now have a powerful weapon in their ability to track criminals, but at enormous potential cost to citizens' privacy.
The face is not just a name-tag.
It displays a lot of other information—and machines can read that, too.
Again, that promises benefits.
Some firms are analysing faces to provide automated diagnoses of rare genetic conditions, such as Hajdu-Cheney syndrome, far earlier than would otherwise be possible.
Systems that measure emotion may give autistic people a grasp of social signals they find elusive.