Facebook is under investigation in the US to determine whether it has "stifled competition and put users at risk".
The new action was announced by New York state Attorney General Letitia James on behalf of a coalition of representatives from several states.
"Even the largest social media platform in the world must follow the law and respect consumers," said Ms James.
Facebook said people had "multiple choices" when it came to using online services.
"We understand that if we stop innovating, people can easily leave our platform. This underscores the competition we face, not only in the United States but around the globe," said Will Castleberry, Facebook's vice president for state and local policy.
State officials from Colorado, Florida, Iowa, Nebraska, North Carolina, Ohio and Tennessee are part of the investigation, as well as the district of Columbia.
"We will use every investigative tool at our disposal to determine whether Facebook's actions may have endangered consumer data, reduced the quality of consumers' choices, or increased the price of advertising," said Ms James.
Facebook already faces a separate anti-trust investigation by the US Federal Trade Commission.
And in July, the US Justice Department opened a broad investigation of major technology firms, focusing on potentially anti-competitive practices.
Facebook has previously claimed it is not a monopoly and said consumers can choose how to connect with friends online.