WASHINGTON — In a stunning setback to regulators’ efforts to break up Facebook, a federal judge on Monday threw out antitrust lawsuits brought against the company by the Federal Trade Commission and more than 40 states.

The judge eviscerated one of the federal government’s core arguments, that Facebook holds a monopoly over social networking, saying prosecutors had failed to provide enough facts to back up that claim. And he said the states had waited too long to bring their case, which centers on deals made in 2012 and 2014.

The judge said the F.T.C. could try again within 30 days with more detail, but he suggested that the agency faced steep challenges.

The rulings were a major blow to attempts to rein in Big Tech. In Congress, legislators pointed to the decisions as proof that century-old antitrust laws needed updating for the internet sector.

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The decision was a disappointment for the growing cohort of activists who have pushed regulators to move to break up the biggest tech companies. Sarah Miller, the executive director of the antitrust think tank the American Economic Liberties Project, said she hoped the states would appeal the dismissal and that the F.T.C. would file its case again. But she said the ruling underscored the need for Congress to update the laws that police market concentration.

“The courts are going to need, ideally, some congressional guidance here, given that they have some outsized role in determining the outcomes of antitrust cases,” Ms. Miller said. “Sometimes losses can be good because it can just reinforce that necessity, and we’re hoping that this will serve to do that.”