SAN FRANCISCO — In a decision fleshing out the rights of accused criminals in the social media age, a California appeals court ruled Tuesday that Facebook, Twitter and Instagram don’t have to honor subpoenas from criminal defendants seeking private user information.

The First District Court of Appeal found that, although prosecutors with a warrant can mine social media sites for evidence to put on their case, federal Internet privacy laws prevent criminal defendants from doing the same. Justice Terence Bruiniers, writing for a unanimous panel, found that quashing the subpoenas of two men charged with murder was required by the federal Stored Communications Act and would not jeopardize their constitutional right to a fair trial. But the justices made clear that their opinion only applies to pretrial discovery, leaving the door open for the defendants to try again during their upcoming trials.