"We stand at a moment when the very issue at the heart of this case—the interplay between private entities, nontraditional media, and the First Amendment—has been playing out in the courts, in other branches of government, and in the media itself," attorneys for MNN wrote in their final plea to the justices to take up the case.

A ruling against MNN on the broad question it has asked the court to consider could open social media companies to First Amendment suits, which would force them to limit the actions they take to control the content on their platforms.

The court could also rule more narrowly against MNN in a way that does not impact the companies.

The case is likely to get extra attention as it moves forward given Republican lawmakers' increasing attacks against social media companies for perceived partisanship. Those attacks have raised the specter that the court, which has served as a bulwark for conservative expression, could step in.

Some observers have expressed caution, saying that the justices are unlikely to rule in a way that could substantially impact social media companies.

Michael Pachter, a former tax attorney who covers Twitter as an analyst at Wedbush Securities, said he thought it was "extremely unlikely" that the court will issue a ruling that hamstrings social media companies, particularly given the court's deference to business interests.

If the court does place serious limits on how the companies can restrict the speech on their platforms, he said, it would make the networks more hostile, alienating their users and advertisers.

"[Twitter] is an uncivil place as it is," Pachter said. "But it will become less civil."

Courts in California and New Jersey have weighed in on the issue, finding that social media companies don't constitute state actors subject to First Amendment liability. A federal judge in New York ruled in May that the First Amendment protected users interacting with parts of Twitter, including the president's feed. But that ruling did not apply to Twitter as a whole.

The Supreme Court addressed a related issue in June 2017. In Packingham v. North Carolina, No. 15-1194, the court struck down a state law that prohibited sex offenders from accessing social media sites. In his opinion for the court, Justice Anthony Kennedy, who retired over the summer, referred to social media sites as a "modern public square." But the court's decision left important questions about what exactly that meant up in the air.