Here's a self-explanatory quote from Boudwin v. General Ins. Co. of America, 2011 WL 4433578 (La. App. Ct. Sept. 14, 2011)--
On cross examination, however, Lee acknowledged several entries from his Facebook page where he reported frequently “working out” and also playing sports such as basketball, tennis, “ultimate Frisbee,” and softball, sometimes engaging in multiple sessions of sporting activities in a single day. He further acknowledged that he wrote on his Facebook page that he had participated in a softball tournament in the month before trial, which happened to be two days before his final visit with. Dr. Cenac. When asked if he had informed Dr. Cenac of any of injuries he had sustained while playing sports, he stated that he told Dr. Cenac that he stayed “active,” but that he was “not inclined” to tell Dr. Cenac that he was playing on softball teams.
Read commentary about the case, and further examples of social media tripping up litigants, at Goldman's Observations.
Related post at Trial Practice Tips: "Facebook and Discovery, an Update."