Senators Charles E Schumer of New York and Richard Blumenthal of Connecticut confirmed yesterday that they have written to the US justice department and the Equal Employment Opportunity Commission, asking them to investigate the practice.
This follows recent reports from both the US and UK of private and public employers asking job seekers to present their social media information at the interview stage.
The practice has been widely criticised by privacy campaigners, but its legality remains uncertain.
In a statement, Schumer said: "Employers have no right to ask job applicants for their house keys or to read their diaries. Why should they be able to ask them for their Facebook passwords?"
In response to the controversy, Facebook last Friday urged people never to share their passwords with anyone, and warned employers that they may risk breaking discrimination laws by viewing information on Facebook pages before deciding whether to hire someone.
Personal information such as gender, age, race and religion are all often displayed on Facebook profiles, and these are also details that are protected by US and UK employment law.
Mark Zuckerberg's company further threatened legal action against any third-party applications that violated its longstanding policy against sharing passwords.
The senators want to know whether the process of employers requesting passwords or personal information violates the Stored Communications Act or the Computer Fraud and Abuse Act.
These two pieces of US legislation prohibit intentional access to electronic information or someone's personal computer without prior authorisation.
Schumer said in the letter: "In an age where more and more of our personal information - and our private social interactions - are online, it is vital that all individuals be allowed to determine for themselves what personal information they want to make public and protect personal information from their would-be employers.
"This is especially important during the job-seeking process, when all the power is on one side of the fence."
Schumer and Blumenthal further want to know if court cases relating to employers asking current employees for their social media credentials could be applied to job applicants.
Such cases include former human resources executive John Flexman starting tribunal proceedings against gas exploration firm BG Group after accusing it of forcing him out for putting his CV on business network site LinkedIn.
Flexman is claiming thousands of pounds in compensation from BG Group. The case is due to be heard later this year.
There have been more reports of employers requesting social media credentials in the US, but it is expected that the practice will spread more to the UK soon.
Sarah Veale, the head of equality and employment rights for the TUC, told The Daily Telegraph: "Once something like this starts happening in the US, it is likely to come over here - especially in American businesses which have outposts in UK.
"If interviewers in the US are adopting this practice of asking prospective staff for access to their Facebook accounts, they will start doing it over here."
Veale said that it is "dangerous and unnecessary" for employers to ask people for "access into their personal lives".
"Once you start asking people to reveal everything about themselves, which is irrelevant to their ability to be able to do a job, you are getting into a tricky area," she said.
"It's the equivalent of getting people to spy on prospective staff down at the pub before hiring them.
"It's also quite a lazy way by bosses to get a full picture of somebody and shows that their interviewing process is unsatisfactory."