This story out of the UK highlights the dangers of repeating libelous content on social media.
In the midst of an ongoing sexual abuse scandal revolving around senior figures in the media and accusations of coverups at the BBC, the news program Newsnight claimed a "senior Conservative politician" was involved in the abuse of children at an orphanage. Another program claimed to have a list of names. Soon those names leaked onto Twitter, and Lord McAlpine (a member of Margaret Thatcher's cabinet) found himself wrongfully identified as one of the abusers. The accusation was retweeted over and over.
Lord McAlpine didn't take this lying down. He has threatened to sue anyone who retweeted the rumour unless they make a public apology, and has extracted apologies and payment of damages from the BBC and others.
While libel laws in the UK may make going after Twitter users easier in that country, this is still a cautionary tale for users of social media. There can be serious consequences if you aren't careful what you say or repeat online. For more on the potential pitfalls of using social media, see LAWPRO's article on Social Media: Pitfalls to Avoid.