Google and Facebook have options called photo tagging. When someone posts a picture, then names you as being in the picture, you have been tagged. Behind this is new cutting-edge facial-recognition software to enhance their photo editing and sharing services.
Both firms encourage users to assign names to people in photos. Facial-recognition software then goes to work indexing facial features like a fingerprint expert indexes swirls in a thumbprint. Once you are tagged in a photo, the software looks for similar facial features in untagged photos. This allows users to quickly group photos in which you appear. Google and Facebook say privacy is protected because photo tagging is designed strictly for use by individual consumers within their personal accounts. May be fun, but also scary. Caveat Emptor.