From the page:
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.