California Supreme Court has ruled that websites (like this one) are not liable for libel committed by third parties. The court based it on federal, not state, law:
In a 34-page ruling, the state’s high court overturned a lower court decision that had stripped immunity against such lawsuits and alarmed free speech advocates who warned it could chill expression on the Internet. Despite reservations about offering blanket legal protection for posting defamatory material written by others, the Supreme Court unanimously concluded that federal law is clear on the subject.
‘We acknowledge that recognizing broad immunity for defamatory republications on the Internet has some troubling consequences,’ Justice Carol Corrigan wrote for the court. ‘Until Congress chooses to revise the settled law in this area, however, plaintiffs who contend they were defamed in an Internet posting may only seek recovery from the original source of the statement.’