FACEBOOK shares tumbled nearly five per cent in the US overnight after the Federal Trade Commission confirmed it had opened an investigation into the embattled social network.

According to the New York Post, the FTC is looking into the data practises employed by Mark Zuckerberg’s social network after it was revealed that the private data of 50 million users got into the hands of political consultancy firm Cambridge Analytica, it said in a statement.

“The FTC takes very seriously recent press reports raising substantial concerns about the privacy practises of Facebook,” the statement said. “Today, the FTC is confirming that it has an open non-public investigation into these practises.”

In 2011, Facebook signed a consent decree with the FTC that restricted the ways the company could share users’ personal data. The decree was reached after the agency accused the social network of deceiving its users “by telling them they could keep their information on Facebook private, and then repeatedly allowing it to be shared and made public,” the FTC said at the time.

Among other things, Facebook had told users that third-party apps on its site wouldn’t be able to access their data. Nevertheless, the apps got access to almost all of their personal information, according to the FTC.

Facebook agreed to a $US40,000 ($51,000) penalty for each future violation.

News of the FTC probe sent Facebook shares down 5.9 per cent, to $US149.85 ($194), a more than eight-month low.

It comes a day after the embattled social network bought ads in US and British newspapers to apologise for the Cambridge Analytica scandal, but faced new questions about collecting phone numbers and text messages from Android devices.