1. One or more computer-readable storage media that store executable instructions to identify a first face, wherein the executable instructions, when executed by a computer, cause the computer to perform acts comprising: receiving, from a first user, a first image that contains said first face; for each of a plurality of second users, determining how many times said first user has uploaded images of said second users; for each of said plurality of second users, calculating distance metrics that represent visual differences between said first face and faces associated with said second users; based on how many times said first user has uploaded images of said second users, and also based on how different said first face is from faces associated with said second users, identifying a third user whom said first face is a picture of; tagging said first face as being a picture of said third user; and converting, into first probabilities, numbers of times that said first user has uploaded images of said second users, such that, for any pair of said second users, the user in said pair whose face has been uploaded by said first user a larger number of times has a higher probability of being the user that said first face is a picture of; wherein said identifying of said third user is based on said first probabilities.