Data misuse: 'Socially responsible' investors reassess Facebook ownership

With European and US lawmakers calling for investigations into reports that Facebook user data was accessed by UK based consultancy Cambridge Analytica to help President Donald Trump win the 2016 election, investors are asking even more questions about the social media company's operations.

An increasingly vocal base of investors who put their money where their values are had already started to sour on Facebook, one of the market's tech darlings.

Facebook's shares closed down nearly 7.0 per cent on Monday, wiping nearly $40 billion off its market value as investors worried that potential legislation could damage the company's advertising business.

Facebook Inc Chief Executive Mark Zuckerberg is facing calls from lawmakers to explain how the political consultancy gained improper access to data on 50 million Facebook users.

Cambridge Analytica said it strongly denies the media claims and said it deleted all Facebook data it obtained from a third-party application in 2014 after learning the information did not adhere to data protection rules.

"The lid is being opened on the black box of Facebook's data practices, and the picture is not pretty," said Frank Pasquale, a University of Maryland law professor who has written about Silicon Valley's use of data.

The scrutiny presents a fresh threat to Facebook's reputation, which is already under attack over Russia's alleged use of Facebook tools to sway US voters with divisive and false news posts before and after the 2016 election.