The probe by the states will look into whether Facebook "endangered consumer data, reduced the quality of consumers' choices, or increased the price of advertising," said New York Attorney General Letitia James in a statement.
The antitrust investigation is part of a larger review that's being coordinated with attorneys general in Colorado, Florida, Iowa, Nebraska, North Carolina, Ohio, Tennessee and the District of Columbia.
"Even the largest social media platform in the world must follow the law and respect consumers," James said.
This is all happening at a time when Facebook has agreed to pay $5 billion to the Federal Trade Commission for failing to protect user data from broad sharing with third-party apps, among other problems.
And the Justice Department said earlier this summer that it's launching a similarly wide-ranging antitrust review of big tech companies. The DOJ didn't name specific firms in its announcement, but it was the first clear public confirmation of a major U.S. antitrust review of the tech industry.
Alphabet, Google's parent company, on Friday said in a regulatory filing that the DOJ has requested "information and documents relating to our prior antitrust investigations in the United States and elsewhere." Alphabet said it expects to receive similar requests from state attorneys general.
"We continue to cooperate with the DOJ, federal and state regulators in the United States, and other regulators around the world," the company said.
The Justice Department said its antitrust division will look at how major online platforms have grown their share of market power and whether they are acting in ways that have "reduced competition, stifled innovation, or otherwise harmed consumers."
A request for comment from Facebook was not immediately returned.
Editor's note: Facebook and Google are among NPR's financial supporters.