Several US state attorneys general will investigate Facebook on whether the social network stifled competition and put users at risk.
It comes days after reports that Google would face an antitrust probe from a host of attorneys general.
Technology companies are facing a backlash in the United States and across the world, fueled by concerns among competitors, lawmakers and consumer groups that the firms have too much power and are harming users and business rivals.
Facebook, which owns one-time rivals Instagram and WhatsApp, and has more than 1.5 billion daily users, has a huge influence in many countries and has been criticized for allowing misleading posts and so-called "fake news" on its service.
Google has faced accusations that its web search service, which has become so dominant that it is now a verb, leads consumers to its own products at the cost of competitors.
"I'm launching an investigation into Facebook to determine whether their actions endangered consumer data, reduced the quality of consumers' choices, or increased the price of advertising," tweeted New York AG Letitia James, who is leading the probe by a bipartisan coalition.
Attorneys general investigating the company also include those of Colorado, Florida, Iowa, Nebraska, North Carolina, Ohio, Tennessee and the District of Columbia, according to a statement.
The New York attorney general's office said it expected more states to join the investigation.
State attorneys general often team up to influence policy, including when the federal government lacks the resources or is yet to act. They may also try to enforce state antitrust or consumer laws that provide greater protections than federal law.
The attorneys general of 20 states and the District of Columbia teamed up with the U.S. Department of Justice in 1998 to sue Microsoft Corp for trying to extend its monopoly in personal computer operating systems to internet browsing software. That case later settled.
More recently, 43 states and Puerto Rico sued Teva Pharmaceutical Industries and 19 other drugmakers in May, accusing them of scheming to inflate prices and reduce competition for more than 100 generic drugs.
Facebook's shares were down 2% in early trading.
