Some guy by the name of Samy wrote a social network popularity worm for MySpace blogs and made over a million new friends in under 24 hours. The worm also added the text "but most of all, samy is my hero" in the infected users self-description / hero section. The worm source code can still be seen in the Google Cache (just search for Samy's user id 11851658 to jump to the relevant part).

The outstanding story of the creation and spread of a MySpace worm.

Update: The folks at Google Blogoscoped have interviewed Samy.