Facebook founder and CEO Mark Zuckerberg arrives to testify following a break during a Senate Commerce, Science and Transportation Committee and Senate Judiciary Committee joint hearing about Facebook on Capitol Hill in Washington, DC. Saul Loeb | AFP | Getty Images

Facebook's expected settlement of between $3 billion to $5 billion with the Federal Trade Commission (FTC) is a "bargain" that does not go far enough in holding the social media company accountable, two top U.S. senators said in a letter Monday. Sen. Richard Blumenthal (D-Conn.) and Sen. Josh Hawley (R-Mo.), both members of the Senate Judiciary Committee, criticized the FTC's investigation of Facebook, saying it's time for the agency to learn from "a history of broken and under-enforced consent orders." The bipartisan letter puts additional pressure on the regulator as it weighs steps to punish Facebook for mishandling users' personal information. "The public is rightly asking whether Facebook is too big to be held accountable," the senators wrote. "The FTC must set a resounding precedent that is heard by Facebook and any other tech company that disregards the law in a rapacious quest for growth."

The FTC launched an investigation into the tech giant last year after the Cambridge Analytica data scandal, which revealed Facebook had improperly shared the data of more than 80 million of its users. The probe centers around whether Facebook violated a 2011 consent agreement with the FTC that requires the company to get explicit permission from users before sharing their personal data. Facebook said in its earnings report last month it had set aside $3 billion for a charge from the FTC, which the tech company said could end up being as high as $5 billion. In the letter, the senators wrote fines alone are "insufficient" and "have done little to deter large tech firms." "We are deeply concerned that one-time penalties of any size every few years are woefully inadequate to effectively restrain Facebook," they wrote.