The US Federal Trade Commission and nearly every US state has sued Facebook Inc, saying it broke antitrust law and should potentially be broken up.

With the filing of the twin lawsuits, Facebook becomes the second big tech company to face a recent major legal challenge in the United States.

The FTC said in a statement on Wednesday that it would seek an injunction that “could, among other things: require divestitures of assets, including Instagram and WhatsApp”.

In its complaint, the coalition of 46 states, Washington, DC and Guam also asked for Facebook’s acquisitions of Instagram and WhatsApp to be judged to be illegal.

The US Justice Department sued Alphabet Inc’s Google in October, accusing the $US1 ($A1.3) trillion company of using its market power to fend off rivals.

In congressional testimony earlier this year, Facebook Chief Executive Mark Zuckerberg defended much-criticised acquisitions like Instagram and WhatsApp, saying his social media platform helped them expand from small, insignificant companies into powerhouses.

He has also argued that Facebook has a range of competitors, including other tech giants.

The lawsuits are the biggest antitrust cases in a generation, comparable to the lawsuit against Microsoft Corp in 1998.

The federal government eventually settled that case but the years long court fight and extended antitrust scrutiny prevented the company thwarting competitors and is credited with clearing the way for the explosive growth of the internet.

Facebook shares fell as much as 3 per cent after the news before paring losses and were last down 2 per cent before close of trade.