If you get an email on Facebook with a subject line something like “FBI vs. Facebook” do not click on the link contained therein. from Snopes.com:
Clicking through the link will initiate the download of a malicious executable (fbi_facebook.exe) file onto recipients’ PC.
The virus is not new, but the email is. Be warned. As always, don’t open emails from folks you don’t know. Just don’t.