Facebook Notes allows users to include tags, Facebook crawls the image from the external server and caches it. Facebook will only cache the image once, but the cache can be by-passed and the feature can be abused to cause a huge HTTP GET flood. The target server is observed to have massive http get flood. Thousands of get request are sent to a single server in a couple of seconds. Total number of facebook servers accessing in parallel is 100+ (100+)”]
Source: http://chr13.com/2014/04/20/using-facebook-notes-to-ddos-any-website/