Tumblr was infected with a malicious worm that spammed user's Tumblr feeds with a vitriol-filled anti-Tumblr message. A trolling group called GNAA took responsibility for the hack saying that it warned Tumblr about the exploit it used weeks in advance and that the company was lucky all it did was spread spam. It also told Gawker that it was planning a "War on Blogs." Whatever that means.
5. Tumblr Hack