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From the makers of the online calendar 30Boxes comes fbExchange, an ad exchange for Facebook apps. In short, this lets application developers add a text link ad to their apps. In exchange, they get referred traffic from other applications on the service. I guess you could call it a "Facebook ring".
Among those Facebook apps already signed up: Free Gifts, Twitter and slideshow site Slide, whose offerings include the number one application on Facebook, Top Friends. In addition to traffic swapping, application developers can reserve impressions and clicks for paid placements and earn money.