Koobface, whose name is an anagram of the word "Facebook", is a special worm that spreads through messages in social networking sites like Facebook and myspace. One of the ways it affects users is:
1. One of friends writes a message on your wall with a video link saying "Funniest video ever!" or "My Home Film;)"
2. When you click on the link, it asks you to install the "Adobe flash player" to see the video.
3. You download it and become a zombie.
More on KoobFace:
The Real Face of KOOBFACE, analysis by Trend Micro.
Koobface virus hits facebook
Koobface scam by Berkeley researchers
Wiki has some good links