At the Information Security Conference in Taiwan this week, researchers from the Foundation for Research and Technology Hellas (FORTH) in Greece will present details of an experiment that involved enlisting Facebook users in a potentially devastating kind of Internet attack. The researchers created an application that displays photographs from National Geographic on a user's profile page. However, invisible to the user, the app also requests large image files from a target server--in this case, a test machine hosted at FORTH. Provided that enough people add the application to their page, the resulting flood of requests can shut down the server or render it inaccessible to legitimate users.
From MIT's Technology Review on social networking and "malicious code".