Recently, a worm was identified as being the culprit behind a rash of Facebook spam messages. The spam involves the distribution of violent or porn-related images. Supposedly, there is little or no interaction required of users for their account to become infected with this worm. If you receive any messages with this type of image, flag it to Facebook and do not click on it.
Also, you will need to change your password. Users with weak passwords are the ones who have been targeted.
CNN posted an article on this threat today: http://www.cnn.com/2011/11/15/tech/social-media/facebook-porn-spam/index.html?hpt=hp_t3