For a long time, public posts on the Internet have been admissible as evidence. But more and more often, private or restricted posts are being subpoenaed from sites like Facebook and MySpace for use in court.
From the article:
In the United States, postings on social networks are generally governed by the federal Stored Communications Act, which regulates how private information can be disseminated in non-criminal matters. The law has been interpreted to mean that the sites don’t have to hand over users’ personal data in response to a civil subpoena. Defense lawyers, though, have devised a strategy to work around this roadblock: They ask judges to order plaintiffs to sign consent forms granting defendants access to their private material. The defendants then attach these consent forms when they subpoena the sites. In these subpoenas, the plaintiffs are essentially authorising the sites to hand over printouts of the private portions of their pages to the defendants.
Long story short – if you’re going to claim a debilitating injury, you probably shouldn’t post photos of your rock-climbing trip a week later on Facebook. Even if they’re “private”, they’re not.