According to Slate's L.V. Anderson, although the laws are different between the United States and the United Kingdom, in either place you can be sued for committing libel based on what you write on Twitter and Facebook.
Medium of communication is irrelevant in American defamation laws; even an email sent to a single person can be libelous. To be libelous (in the United States), a statement must be false and damaging to an individual or corporation, and the person who made that statement must have been at fault (i.e., known that the statement was false, acted recklessly with regard to the facts, or otherwise been irresponsible). Whether a person makes a defamatory statement on a blog, in a newspaper, or on Twitter or Facebook, he or she can be held legally liable for it.
In the United States, however, if you retweet a libelous statement, you are unlikely to be sued for damages. That's because of Section 230 of the Communications Decency Act, which states that "No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider" with regard to defamation and invasion-of-privacy cases. A simple retweet or Facebook "like" is likely to be protected under Section 230 -- but a modified tweet or Facebook comment could constitute libel.
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