Most of the law regarding discovery in criminal cases focuses on what the prosecution must disclose to the defense. After all, the prosecution is the side with all the information and resources. But, at least in some sense, discovery is a two-way street: Defendants have to turn over certain information to the prosecution.
In a criminal case, the prosecution has to disclose certain information that’s helpful to the defense. Perhaps just as importantly, though, it also has to hand over information that hurts the defense—this is the evidence that the prosecution plans to rely on to prove guilt.
Prosecutors don’t have to actively search for or follow every lead that might lead to information that could help a defendant. But they do have to turn over information that’s favorable to the defense, even if the defense doesn’t ask for it.
When people update their status or share photographs on Facebook and similar social networking sites, the justice system isn’t usually on their mind. But law enforcement officers who troll these sites for information related to investigations often find fruitful information for the prosecution in a criminal prosecution. Since the information is generally public, there’s nothing to stop the prosecution from using it against the defendant at trial. And even if prosecutors don’t use it in the courtroom, it might provide investigative leads.