A wonderfully inciteful article on face blindness. I first came across this interesting condition by reading the works of Sacks. This piece captures how crucial the social ability to recongise other people truly is.
Prosopagnosia or face blindness is a problem with how the brain processes faces rather than the eyes. This strange ailment causes the sufferer to forget faces, as if they cannot store the collected details of a face and thus while they don't forget the person they can't remember the face.
Certain coping mechanisms are used, such as trying to remember a beard or glasses or any other details that might aid the sufferer in remembering the individual.
Sacks describes these coping mechanisms with his own conversation of how he suffered with prosopagnosia.
Prosopagnosia and Neurology