Last week, the California Supreme Court ruled that social media firms like Facebook, Twitter, and Instagram must turn over public posts by users to criminal defendants. This is an important decision that can impact thousands of cases.

For years, social media companies have argued that the Federal Stored Communications Act bars them from sharing electronic communications except in limited cases, such as in response to a search warrant. The social media companies also argued that the defendants had other ways to obtain the information.

The defendants seeking the information were Derrick Hunter and Lee Sullivan. The two men were charged with murder. The information sought was contained on the social media accounts of the state’s star witness and the victim. Hunter and Sullivan argued that their constitutional right to a fair trial entitled them to the information on social media. Hunter and Sullivan explained that there was exculpatory evidence on Instagram, in particular.

The trial judge had found that the defendants’ constitutional rights were at issue and ordered the social media companies to produce the information for the court’s review. The social media companies appealed the ruling and won at the California state appeals court. That ruling prompted defendants to take an appeal to the California Supreme Court.

The California Supreme Court noted that all parties agreed that if a user made a post public, that was consent for it to be shared. However, the court rejected defendants’ view that postings to large groups of friends were public, but also noted that the Federal Stored Communications Act did not permit the social media firms to ignore subpoenas. The state supreme court remanded the case to the trial court to essentially decide what is a public post and what is private. The trial court will also have to decide how to handle posts or tweets that were once public, but later deleted or made private.

Until now, courts have routinely quashed subpoenas to social media firms seeking this type of information. This decision opens the door for criminal defendants, including those fighting health care fraud prosecutions, to mine social media for potentially valuable exculpatory evidence about Government witnesses.