Social networking sites like Facebook and MySpace give scam artists and virus writers new ways to package tried-but-true tricks. The latest example of this making the rounds is an e-mail that appears to be an invitation from Facebook to add a friend: A recipient who opens an attached image to take a look at their new friend instead opens the door for hackers to compromise his PC. Internet security firm Websense warns about this latest scam, which takes advantage of common notifiers sent by Facebook to alert users when another user adds them as a friend on their social network: The spammers included a zip attachment that purports to contain a picture in order to entice the recipient to double-click on it. The attached file is actually a Trojan horse. The message also includes a login form to the Facebook home page. While there are countless examples of scam e-mails