Vice Chairman of the Senate Select Committee on Intelligence and co-chair of the Senate Cybersecurity Caucus, Warner said it was high time the Congress stepped up and took action to protect privacy and security of social media users.

"The news that at least 50 million Facebook users had their accounts compromised is deeply concerning. A full investigation should be swiftly conducted and made public so that we can understand more about what happened," the Democrat said in a statement late on Friday.

In the biggest-ever security breach after Cambridge Analytica scandal, Facebook on Friday admitted hackers broke into nearly 50 million users' accounts by stealing their "access tokens" or digital keys.

This allowed them then use the tokens to take over people's accounts.

Warner said the disclosure is a reminder about the dangers posed when "a small number of companies like Facebook or the credit bureau Equifax are able to accumulate so much personal data about individual Americans without adequate security measures".

"As I've said before - the era of the Wild West in social media is over," he said in the statement.

Facebook also said it was taking precautionary step to reset access tokens for another 40 million accounts that have been subject to a "View As" look-up in the 2017.

As a result, around 90 million people will now have to log back into Facebook, or any of their apps that use Facebook login.

Indian-origin FTC Commissioner Rohit Chopra also tweeted on the incident, saying "I want answers".

In July, Warner published a policy paper, outlining why regulation is necessary for social media companies.

Reacting to the new data breach, CEO Mark Zuckerberg said "While I'm glad we found this, fixed the vulnerability, and secured the accounts that may be at risk, the reality is we need to continue developing new tools to prevent this from happening in the first place."

Zuckerberg and Facebook COO Sheryl Sandberg have faced a couple of hearings at US Congress over Cambridge Analytica data breach that affected 87 million users.

Facebook has also admitted that the phone numbers that its users provide for security purposes were being used to target them with ads.

"We use the information people provide to offer a better, more personalised experience on Facebook, including ads," a Facebook spokesperson was quoted as saying by TechCrunch on Thursday.

The social network specifically uses a phone number that users provide for two factor authentication (2FA) -- a security technique that provide a second layer of authentication to help keep accounts secure, the report added.