Find untagged photos of your Facebook friends with Face.com’s Photo Finder

This one is kinda scary because of how well it works. Face.com‘s new Photo Finder application for Facebook helps you automatically discover public photos that you and your friends may have forgotten to tag — and it also lets you track untagged photos of your friends.

Face-recognition technology is itself not new, but Photo Finder’s twist is how it makes use of Facebook’s interface. The social network only shows you photos of yourself containing tags about you — your name and profiled, associated with you in a given photo. Up until now, untagging a photo is how you hide a photo from your Facebook friends — the other option is to use more advanced privacy settings that restrict photo viewing to specific friends, but I’m not sure how many people use that feature.

With Face.com, your friends can bypass such social engineering to directly stalk you, or visa versa as the case may be.

First you add Photo Finder and give it access to your photos, friends and other social data, then it uses this information to identify which photos might belong to whom. In the site interface, you can see a “home” view that shows you a random selection of photos of you and your friends. The “me” section shows you any photo possibly of you. The “friends” section lets you search through photos of your friends. The “watch list” — the really stalkery part — lets you track the photos of individuals who you select from your friends list. The “who’s this?” section shows you a random assortment of photos from your friends photo albums, and asks you to help identify people in those photos. You can sort photos by date, accuracy, and see which ones have Facebook tags or not.

In my testing so far, the site has done an impressive job of finding photos of myself that I’ve tried to hide (thanks?). There’s a couple from a friend’s birthday party from last year, another few from a more recent tech conference. Load time is sometimes slow, and its not always completely accurate — but I’ve never seen face recognition software that works this well for Facebook photos.

More than 850 million photos are uploaded every month by Facebook’s 200 million or so users. So if this app becomes popular, expect a surge in the number of these photos that people choose hide behind the more advanced privacy setting. I’m guessing that college students coming back from spring break, for example, might think harder about making a new album like “Beach Party Spring Break 09″ available to just a few friends or a network — but not moms or ex-girlfriends.

Companies already employing facial recognition technology range from the U.S. State Department (for visa ID tracking) to MyHeritage’s genealogy site (see how similar you look to a celebrity). Aside from Facebook customization, Face.com thinks its technology will allow it to effectively deploy this technology across millions of Facebook users and their millions of photos.

The application is currently in private testing, but we have some invites that you can use to check the site out here.