As I preface every Facebook post, it is difficult to tell how widespread a feature (or test) is because they are constantly testing new products to different segments of their massive audience. With that out of the way, I have started to see image tagging units in the advertising panel on Facebook image pages. It probably appears on <5% of impressions. It shows an answer with someones face and asks you to tag it. My guess is that it is a mechnical turk like confirmation of friend tags, or more far fetched, an experiment to confirm their facial recognition
But my first reaction was actually neither of these things. It was a recollection of Google's Image Labeler game / tool from a few years ago. Speaking od game mechanics and Mechanical Turk - Image Labeler was ahead of the curve.
Google would have you paired with another user in real time. The goal of the game is to both type the same word - any descriptive word- about a series of images. Upon user pairing, it would reveal an image synchronously and a list of obvious words that you can not use in the game. Once you both enter the same descriptive word not on the banned list - points are awarded (function of speed, accuracy) and the next image is loaded.
Its simple to play. Its synchronous. Points are awarded and leader boards are displayed throughout the experience - so its competitive. And it assists Google and their various image / algorithm / matching technologies.