There is a new facebook application doing the rounds by the name of Photas, it will say that a frnd of urs commented on a photo of you, and when u try to check the photo, it will take u to this page: http://www.facebook.com/apps/application.php?id=448829670716 , goign there will send this trojan to all your friends and thus spread exponentially.
Do not fall for this.
In General, dont take everything for granted on sites like facebook etc, look before you add apps, u may never know what you might give away.
Forward this to your friends so that they also dont fall for this.