Update 5.47pm: The chairman of the British parliamentary inquiry into fake news has threatened Facebook founder Mark Zuckerberg with a formal summons to give evidence before the inquiry in person.
The move came after an "unsatisfactory" hearing with another Facebook executive on Thursday.
In a statement following nearly five hours of testimony from Facebook's chief technical officer Mike Schroepfer, chairman Damian Collins said Mr Schroepfer "failed to answer many specific and detailed questions" about the company's role in a number of ongoing scandals about how political campaigns use the platform to influence voters.
"As an American citizen living in California, Mr Zuckerberg does not normally come under the jurisdiction of the UK Parliament, but he will the next time he enters the country.
"We hope that he will respond positively to our request, but if not the committee will resolve to issue a formal summons for him to appear when he is next in the UK," Mr Collins said.
The statement issued by the UK's select committee for Digital, Culture, Media and Sport accused Mr Schroepfer of failing to answer "nearly 40 separate points", about how Facebook handles users' data and interacts with political campaign groups.
Update 5pm: The DCMS select committee has released a statement calling Mike Schroepfer, Facebook's chief technical officer's testimony "unsatisfactory", citing nearly 40 occasions it says the Facebook executive failed to answer questions sufficiently.
Committee chairman Damian Collins reiterated his request for Facebook founder Mark Zuckerberg to give evidence to the committee in person, threatening to issue a formal summons for him to appear before MPs the next time he visits the UK.
Chair @DamianCollins tells @facebook's Chief Technical Officer, Mike Schroepfer, about our concerns around the use of microtargeting for political elections and campaigns as part of our broader inquiry into #FakeNews.April 26, 2018
He said in a statement: "Mr Schroepfer, Mark Zuckerberg's right-hand man whom we were assured could represent his views, today failed to answer many specific and detailed questions about Facebook's business practices.
"We will be asking him to respond in writing to the committee on these points; however, we are mindful that it took a global reputational crisis and three months for the company to follow up on questions we put to them in Washington DC on February 8.
"We believe that, given the large number of outstanding questions for Facebook to answer, Mark Zuckerberg should still appear in front of the committee. We note, in particular, reports that he intends to travel to Europe in May to give evidence to the European Parliament and will request that he appears in front of the DCMS committee before the 24th May.
"There are over 40 million Facebook users in the UK and they deserve to hear answers from Mark Zuckerberg about the company he created and whether it is able to keep their users' data safe.
"As an American citizen living in California, Mr Zuckerberg does not normally come under the jurisdiction of the UK Parliament, but he will the next time he enters the country. We hope that he will respond positively to our request, but if not the committee will resolve to issue a formal summons for him to appear when he is next in the UK."
After Mike Schroepfer from @facebook's evidence today, the @CommonsCMS and @DamianCollins believes that the number of outstanding questions that remain warrants an appearance by their CEO Mark Zuckerberg. Read our full statement here >> https://t.co/0qvJ1IpoWl pic.twitter.com/r0iclzEkbm— Digital, Culture, Media and Sport Committee (@CommonsCMS) April 26, 2018
Dominic Cummings, former campaign director for Vote Leave, said Mr Schroepfer's evidence "proves exactly" that the "conspiracy theory" of co-ordination between Vote Leave and Leave.Eu via Cambridge Analytica and Aggregate IQ is "factually wrong".
Writing on his personal blog, Mr Cummings said: "No reasonable person could think that the battle between Vote Leave/me and Leave.EU/Banks to control the official campaign really was a deep cover operation to hide our secret co-ordination over data.
"There are serious issues concerning data, marketing and elections as I said before this conspiracy theory got going. It would be much better for the media to focus on these issues than persist Trump-like with claims that black = white."
Earlier: Facebook chief technical officer says he is 'disappointed' with handling of political advertising
Mike Schroepfer, Facebook's chief technical officer, has told the UK Digital, Culture, Media and Sport (DCMS) select committee that he is "disappointed" by the social media giant's handling of Russian disinformation campaigns on the platform.
Facing a grilling about political advertising by committee chair Damian Collins, Mr Schroepfer said: "We were slow to understand the impact at the time and I am way more disappointed in this than you are."
Amid laughter, Mr Collins told Mr Schroepfer "It's a high bar".
Facebook CTO Mike Schroepfer appears before the House of Commons committee https://t.co/wiZkVhQ793— Reuters UK (@ReutersUK) April 26, 2018
"I'm sorry, I shouldn't have said that," the Facebook executive replied. "It's something we're working very hard on."
Mr Schroepfer is answering questions about how malicious actors used Facebook to impersonate people and attempt to influence elections with political advertising.
Mr Schroepfer stressed that political advertising on Facebook is "a very small, low, single-digit percentage of our advertising revenue" and the company believes transparency is the best way to protect voters from malicious political ads.
"There are a number of challenges you've raised and we need to do better," he said, but added that, for people outside the political mainstream, the combination of Facebook pages and advertising on the platform is "a powerful tool of free speech".
The solution, he claimed, is greater transparency around who is paying for political adverts on Facebook and making the content of those adverts visible to everyone.
In written evidence provided before the hearing, Mr Schroepfer detailed plans to create a "searchable archive" of political adverts in time for the UK local elections in May 2019.
Facebook CTO Mike Schroepfer says political ads on its platform will be transparent in time for the England and Northern Ireland's 2019 local elections pic.twitter.com/dStp4LxOFG— TicToc by Bloomberg (@tictoc) April 26, 2018
Mr Schroepfer stressed how Facebook is pursuing technological solutions to malicious behaviour on its platform, where the company is "trying to catch these things proactively".
He said: "We want to get to a mode where people reporting bad content of any kind is the defence of last resort and the vast majority of stuff is caught up front by automated systems."
For much of its history, Facebook representatives had stressed a programme of users reporting misbehaviour on the platform - such as illegal activity, hate speech, abuse, nudity and disinformation - rather than actively policing the network themselves.
Turning to the subject of data collected by Cambridge University academic Aleksandr Kogan and given to election consultants Cambridge Analytica, Mr Schroepfer said Facebook received "legal certification" that the data had been deleted in 2015.
Asked by MP Jo Stevens why Facebook did not inform the Information Commissioner of the breach, Mr Schroepfer said the company believed the matter had been resolved.
"A developer had used our platform, collected some data and then resold that data to a third party and that reselling of the data was the issue," he said.
"Our priority right now is to understand what happened and to co-operate fully with the Information Commissioner and others," he added.
Mr Schoepfer disagreed with the assessment of Dr Kogan, who gave evidence to the committee on Tuesday, that Facebook did not have a policy around the collection of users' personal data.
"Our primary product is to help people share with a limited audience... If you want to share with your friends only, that's the primary thing Facebook does.
"If we violate that trust and the data goes somewhere else then that's violating the core principles of our product," he said.
Pressured to apologise for legal letters sent to the Guardian in March threatening to sue the publication if it published a story about the misuse of Facebook data by Dr Kogan, Mr Schroepfer said he believed it was "standard practice" in the UK and the aim was to "correct some facts".
Asked again to apologise, he said: "I am sorry that journalists feel we are trying prevent them getting the truth out. That was not the intention."
"I am sorry that journalists feel that we're trying to prevent them from getting the truth out," Facebook CTO Mike Schroepfer says when asked if Facebook will apologize for sending legal letters to the Guardian for reporting the data breach scandal pic.twitter.com/pJckHWcJpn— TicToc by Bloomberg (@tictoc) April 26, 2018
Julian Knight MP, accused Facebook of "bullying journalists, threatening academic institutions and impeding investigations by legal authorities", insisting that "Facebook is a morality-free zone" and the company is "the problem" at the centre of the myriad scandals involving data and advertising on the platform.
Going further, he asked: "Do you think the time has come for robust regulation?"
"I agree with you on a few key things," Mr Schroepfer replied, explaining that Facebook believes giving users "safety, transparency and control" was central to solving the problems.
"The details of implementing that is where the real work lies," he said.
Turning to the 2016 EU Referendum, Mr Schroepfer said Facebook had found no evidence Cambridge Analytica had spent money on advertising during the referendum.
But Aggregate IQ, a linked company which multiple Brexit campaign groups spent large portions of their campaign budget with, "spent two million dollars on the Brexit referendum in the UK in 2016," he said.
Mr Schroepfer says that data collected by Dr Kogan and passed to Cambridge Analytica was not used by Aggregate IQ to target voters during the EU referendum campaign.
"The campaigns they [AIQ] run are based on email lists... Dr Kogan did not get email lists," he said.
Instead, the company "must have acquired that data from another source", he added.