"The researchers developed an application called "Photo of the Day," which serves up a new National Geographic photo daily. But in the background, every time the application is clicked, it sends a 600 K-byte HTTP request for images to a victim’s Web site.
Those requests, as well as those images, are not seen by someone using Photo of the Day, which the researchers have termed a "Facebot" application. The effect is a flood of traffic to the victim’s Web site, known as a denial-of-service attack." – Content courtesy of PC World – Researchers Build Malicious Facebook Application