WASHINGTON — Meta did not break the law when it bought its nascent rivals Instagram and WhatsApp, a federal judge said Tuesday, handing a major win to the $1.51 trillion company and dealing a blow to the government’s efforts to rein in the power of tech giants.

Judge James E. Boasberg of the U.S. District Court of the District of Columbia said in an 89-page ruling that Meta did not create a monopoly in social networking through the acquisitions and that the market has continued to expand with rivals including TikTok and YouTube. The Federal Trade Commission had sued the company, accusing it of breaking antitrust law by acquiring Instagram and WhatsApp in a “buy or bury” strategy to cement its social networking dominance.

The FTC “continues to insist that Meta competes with the same old r

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