Social websites, such as Facebook, give people the opportunity to post statements in the comfort of their home or office in the illusion of privacy. Yet statements made on Facebook are not private, they are very very public. A false statement posted by someone can not only be hurtful but also damage the other person's reputation.
The law of defamation has been around for centuries to protect innocent people from false public accusations. There is a defamation claim when:
1. There is a false and defamatory statement (a statement which, if believed, injures a person's reputation).
2. The statement was an unprivileged publication to a third party. Privilege exists between two doctors. If your doctor made a false statement about your health, such as you have HIV, to a friend of yours then that would be an unprivileged publication.
3. The defendant is at least negligent (the speaker knew or should have known that the statement was false).
4. The statement is "per se" actionable, such as accusing you of a serious crime, or creates some other special harm.
There is a second claim that can be brought for defamatory statements: a false light claim. A false light invasion of privacy claim arises when the defendant publicizes a matter that places the plaintiff before the public in a false light.