Just wanted to pass along this information.  Just like email viruses, you have to use some common sense with social networking sites. There are viruses designed specifically for them, now.

Facebook, Twitter and LiveJournal recently reported they were the targets of "denial-of-service" attacks. Hackers launch these attacks by infecting hundreds or thousands of so-called "zombie" computers with malicious software. Once the computers are infected, hackers remotely instruct them to simultaneously attack a targeted Web site, flooding it with so much traffic that it becomes inaccessible.

Koobface is another pervasive threat. It began on Facebook, but now targets a host of other social networks. This worm sends fake messages and links to friends, usually encouraging them to watch a video. Depending on the site, the worm can also post infected links on walls and changes the account user's "Status" by modifying the text and adding a link. Read more on our blog: "Koobface: Not Just for Facebook Anymore" 

From Wikipedia:
Koobface, an anagram of Facebook is a computer worm that targets the users of the social networking websites Facebook, MySpace, Bebo, Friendster and Twitter. Koobface ultimately attempts, upon successful infection, to gather sensitive information from the victims such as credit card numbers. It was first detected in December 2008 and a more potent version appeared in March 2009.

Koobface spreads by delivering Facebook messages to people who are 'friends' of a Facebook user whose computer has already been infected. The messages contain innocuous subject headers like "Paris Hilton Tosses Dwarf On The Street", "LOL", "My friend catched [sic] you on hidden cam", and "My home video ", followed by a link. Upon receipt, the message directs the recipients to a third-party website, where they are prompted to download what is purported to be an update of the Adobe Flash player. If they download and execute the file, Koobface is able to infect their system. It can then commandeer the computer's search engine use and direct it to contaminated websites.

Stay safe while you network online
Be skeptical. Treat every social networking link with caution - especially the ones promising a link to a video.  This is particularly true for sites that don't show you the true link (like Twitter) If you don't know the person who sent you the link. Be cautious.

Guard your personal information. Use privacy settings to restrict who can see your sensitive information, or consider omitting all personal information from your profile.

Choose passwords wisely. Use different passwords for each of your sites; select a randomized combination of numbers and letters.

Have antivirus and antispyware protection. Even if you think you're not infected, scan your machine for dormant viruses with a free scan; and protect your PC with an Internet security suite that includes antivirus, antispyware, and firewall technologies.

Always install updates. If you're already using antimalware software, be sure to install updates which include the latest malware definitions; do the same with updates to your operating system.

Remain vigilant. Malware authors are continually writing new programs to avoid detection, so pay close attention to suspicious behavior.