In a case that could determine how much access criminal defendants' get to witnesses' social media accounts pretrial, justices on the California Supreme Court on Tuesday probed a lawyer representing Facebook, Twitter and Instagram on where to draw the boundary between public and private in the digital world.

Under questioning from Justice Mariano-Florentino Cuéllar, Joshua Lipshutz of Gibson, Dunn & Crutcher said that only when users make a post public with no restrictions as to who can view it would there potentially be lawful consent for his clients to hand over data under the federal Stored Communications Act. The SCA governs how communications service providers must handle user data. Although prosecutors with a warrant can mine social media sites for evidence to put on their case, the SCA has so far been interpreted as preventing criminal defendants from gaining pretrial access to witnesses' social media accounts.

But Cuéllar prodded Lipshutz about whether a post visible to all two billion Facebook users should be considered public.

It should not, Lipshutz said. The term “public” under the SCA, he argued, is limited to data that anyone can access. Social media companies, he said, shouldn't engage in an “exercise of arbitrary decisions” to decide at what threshold users have consented to having their data handed over the third parties.

“The line has been drawn by Congress, and Congress says it has to be public, and that means anyone can access it,” Lipshutz said.

Lipshutz said his clients were in much the same position in this case as the post office would be in a case where a letter sent via the mail might contain a key piece of defense evidence.

“You can't go to the post office to ask them for a copy and have them turn it over,” Lipshutz said.

Chief Justice Tani Gorre Cantil-Sakauye, however, quickly noted that was because the post office “didn't have possession of the item.”

“You and your clients have possession of the item,” she said.

The exchange occurred during arguments in Facebook Inc. v. Superior Court of the City and County of San Francisco, an appeal from the First District Court of Appeal which found the SCA bars social media companies from handing over user records to criminal defendants. At the state's high court, the case has drawn amici support from Google for the social media companies and from local and national criminal defense organizations from the defense.

The underlying Court of Appeal decision reversed a ruling from the trial judge who had ordered the social media companies to hand over posts and activity logs from a witnesses' social media counts for in camera review in a homicide case.

Susan Kaplan, the lawyer for defendant Lee Sullivan, said the SCA should be found unconstitutional, since it interfered with her client's ability to put on a full defense and effectively cross-examine a key witness. During Tuesday's arguments, the justices repeatedly tried to get her to back off her constitutional argument and lay out what her client might be entitled to get under the SCA. But Kaplan largely stuck to her request that the court strike the law down.

“We would be the first court to ever say [the SCA is unconstitutional] in a criminal case,” the chief justice said late in the argument.

“In troubled times like these, California should take the lead,” Kaplan said.