The committee, which has been questioning the use of social media to spread misinformation and influence elections, had invited Mr. Zuckerberg, but he declined. The British committee’s 14-month investigation is part of an aggressive campaign by European authorities to clamp down on powerful American tech giants. The probe comes as German antitrust regulators investigate Facebook, others investigate Google, and the European Union prepares to put in place a sweeping new privacy law that sets more restrictions on how companies gather and share data.
“The Europeans are far more alert about this problem and far more willing to tackle it than the U.S.,” Stephan Lewandowsky, a cognitive scientist at the University of Bristol who studies how people process misinformation, said in an interview.
On Thursday, Mr. Schroepfer mimicked the conciliatory tone Mr. Zuckerberg struck before Congress this month, pledging that Facebook would do more to improve. “I agree that it’s a problem and it’s something we’re making good progress on, but we have a lot of work to do,” he said.
The parliamentary committee is attempting to punch above its weight. Working out of small hearing room near Big Ben and Westminster Abbey, the panel is composed of 11 members who are mostly rank-and-file members of parliament. It has a staff of about 10 clerks, and little authority beyond being able to summon witnesses — it can’t write new laws or enforce penalties if it discovers wrongdoing.
Before the current investigation, the panel was probably best known for its inquiry of the British phone-hacking scandal. (James Murdoch famously had a pie thrust in his face during his testimony.)
No projectiles have been heaved during the present investigation, but the committee is making the most of its newfound attention thanks to the Cambridge Analytica scandal.
So far, there has been little of the rancor and political speechifying that takes over high-profile American hearings. The stereotype of British manners was mostly maintained, even during the tensest moments — lots of “please,” “thank you,” and sarcasm.
“I’m delighted to hear you have a head of integrity,” one member noted dryly when Mr. Schroepfer referred to a colleague in charge of ethics. In criticizing the number of fake accounts on Facebook, another lawmaker wondered why he, an overweight middle-aged man, was getting Facebook invitations from attractive young women.
At other points, frustration bubbled over. A member called Facebook a “giant vampire squid,” an unfavorable reference to a financial crisis-era article about Goldman Sachs, and another compared it to cigarettes. Mr. Schroepfer said the comparisons disappointed him.
“It is obvious that this is a serious inquiry and that the committee is doing its homework and asking very detailed questions trying to get to the heart of some complicated problems,” said Justin Hendrix, executive director of NYC Media Lab, a New York-based research group that met with members of the panel this year.
The panel has been led by Mr. Collins, a member of the governing Conservative party. The inquiry started as a broad investigation into fake news and how misinformation spreads, and whether those capabilities influenced the 2016 referendum when Britain voted to leave the European Union.
As the panel heard from more witnesses, it became more focused on how easily user data from Facebook could be accessed and used for political gain.
In a separate interview this week, Mr. Collins talked about his education on the social platform. “I’ve been genuinely surprised by how much data Facebook gathers,” he said, pulling out books he had been reading, including “The People Vs. Tech,” a critical examination of how social media is influencing society, and “Chaos Monkeys,” a memoir by a former Facebook executive.
Mr. Collins’ profile has grown along with the Cambridge Analytica scandal. His committee questioned the company’s chief executive, Alexander Nix, weeks before news broke about its practices. The panel has published audio records in which an executive tied to Cambridge Analytica discusses how the Trump campaign used techniques used by the Nazis to target voters. A webcast of the committee’s hearing with the former Cambridge Analytica researcher Christopher Wylie was so popular it nearly took down Parliament’s website, Mr. Collins said.
Between television interviews this week, Mr. Collins sat with staff watching a small set as a Canadian parliamentary committee questioned an executive from an online advertising company, Aggregate IQ, that had been involved in the Brexit referendum. When Mr. Collins heard a discrepancy in the testimony, he messaged a member of the Canadian panel. “He read out my text during the hearing,” Mr. Collins laughed.
Aware that he doesn’t have the same financial resources as Facebook and other global tech firms, or even as much as his counterparts across the Atlantic, Mr. Collins has been coordinating his investigation with others around the world. He has been in touch with regulators in France, Germany and Ireland, and even as far afield as Singapore.
His efforts come alongside those of other British regulators. The country’s Electoral Commission is looking into whether laws were broken during the Brexit referendum, while Britain’s data privacy watchdog is investigating Cambridge Analytica, Facebook and how social networks are used for political campaigns.
Mr. Collins plans to publish a report with policy recommendations by this summer.
“Many people would look at what’s happened over the past couple of months and say the case for greater regulatory scrutiny of the way the tech companies work would be appropriate,” Mr. Collins said in the interview. “I don’t think you can put that genie back in the bottle now.”
On Thursday, he exited the hearing frustrated, saying Mr. Schroepfer hadn’t fully answered at least 40 questions. He plans to use a “formal summons” to require Mr. Zuckerberg to attend, a rarely deployed legal step.
“As an American citizen living in California, Mr. Zuckerberg does not normally come under the jurisdiction of the U.K. Parliament, but he will the next time he enters the country,” Mr. Collins said in a statement.
That could be soon. The European Parliament has invited Mr. Zuckerberg to testify as early as next month, though a parliament spokesman said it hasn’t been finalized.