Facebook boss Mark Zuckerberg has been called on by a UK parliamentary committee to give evidence about the use of personal data by Cambridge Analytica.
Cambridge Analytica is a London-based consulting firm, under pressure over allegations it uses illegally obtained data and social media manipulation to influence elections.
Both companies are under scrutiny following claims by a whistleblower, Christopher Wylie, who worked with Cambridge Analytica and alleges it amassed large amounts of data through a personality quiz on Facebook called This is Your Digital Life.
He claims that 270,000 people took the quiz, but the data of some 50 million users, mainly in the US, was harvested without their explicit consent via their friend networks.
Mr Wylie said that data was used by the company to psychologically profile people, then deliver tailored material to get them to vote for Donald Trump in the 2016 US election.
The claims have led to calls from the US and EU to investigate.
The firm insisted it followed the correct procedures in obtaining and using data, but it was suspended from Facebook last week.
On its website, Cambridge Analytica said it could provide data and insights to "drive your voters to the polls and win your campaign" and that it played a "pivotal role" in winning US presidential races.
Facebook shares fell by a further 3 percent on Tuesday, following a 6.7 percent drop on Monday which wiped almost $37bn from its market value.
The company will hold an open meeting with its employees later to discuss the matter, tech news website The Verge is reporting.
US President Donald Trump has welcomed any investigation into Cambridge Analytica as "Americans' privacy should be protected", according to a deputy press secretary at the White House.
'I hope that this representative will be you' - UK inquiry head requests Zuckerberg
In a letter to Mr Zuckerberg, Damian Collins, the chairman of the UK's Commons inquiry into fake news, accused Facebook of giving answers "misleading to the Committee" at a previous hearing which asked whether information had been taken without users' consent.
He said it was "now time to hear from a senior Facebook executive with the sufficient authority to give an accurate account of this catastrophic failure of process".
Requesting a response to the letter by 26 March, the MP added: "Given your commitment at the start of the New Year to "fixing" Facebook, I hope that this representative will be you".
His intervention comes after the UK's Information Commissioner Elizabeth Denham said she would be applying to court for a warrant to search the offices of Cambridge Analytica.
Aleksandr Kogan, who created the personality app from which the data had been harvested, is a senior research associate at the University of Cambridge.
A spokesman from the university said they had "sought and received assurances" from Dr Kogan that no university data, resources or facilities were used for his work and they had found no evidence to contradict that.
However, he confirmed the university was writing to Facebook to "request all relevant evidence in their possession".
Meanwhile, Channel 4 News has also broadcast hidden camera footage in which Cambridge Analytica chief executive Alexander Nix appears to suggest tactics his company could use to discredit politicians online.
However, Cambridge Analytica said the report had "grossly misrepresented" the conversations caught on camera.
"In playing along with this line of conversation, and partly to spare our 'client' from embarrassment, we entertained a series of ludicrous hypothetical scenarios," the company said in a statement.
"Cambridge Analytica does not condone or engage in entrapment, bribes or so-called 'honey traps'," it said.
Mr Nix told the BBC's Newsnight programme he felt the firm had been "deliberately entrapped".
How to protect your data on Facebook
There are a few things to be aware of if you want to restrict who has access to your data.
- Keep an eye on apps, especially those which require you to log in using your Facebook account - they often have a very wide range of permissions and many are specifically designed to pick up your data
- Use an ad blocker to limit advertising
- Look at your Facebook security settings and make sure you are aware of what is enabled. Check the individual app settings to see whether you have given them permission to view your friends as well as yourself.
- You can download a copy of the data Facebook holds on you, although it is not comprehensive. There is a download button at the bottom of the General Account Settings tab. However bear in mind that your data may be less secure sitting on your laptop than it is on Facebook's servers, if your device is hacked.
- You can of course, simply leave Facebook, but the campaign group Privacy International warns that privacy concerns extend beyond the social network.
"The current focus is on protecting your data being exploited by third parties, but your data is being exploited all the time," a spokesperson said.
"Many apps on your phone will have permission to access location data, your entire phone book and so on. It is just the tip of the iceberg."