Zuckerberg already faced two days of questioning by lawmakers in the USA, and Tuesday it was the Europeans' turn to get answers on everything from Facebook's readiness for new data laws, known as GDPR, to how the platform combats fake news.
Zuckerberg declined to answer the questions - promising instead to respond to the inquiries in writing.
Zuckerberg said Facebook was strengthening cooperation with national election authorities and trying to introduce more transparency about who is running political advertising.
The European and US inquiries concern the same fundamental questions about the social media giant, triggered by the news that Facebook permitted Cambridge Analytica to harvest the private data of 87 million users and that Facebook failed to take action when it became clear in 2016 that foreign actors were using its platform to spread disinformation and undermine democratic processes.
"Whether it's fake news, foreign interference in elections or developers misusing people's information, we didn't take a broad enough view of our responsibilities", Zuckerberg said. "That was a mistake and I am sorry for it", he said.
As was the case when Zuckerberg testified on Capitol Hill last month, some exchanges included awkward moments. "We've made these kinds of attacks much harder to do on Facebook".
The spokesman said in an emailed statement that "the average person uses eight different apps to communicate and stay connected".
The meeting was streamed on the Parliament website, as well as on its Facebook page.
But, in responding to questions, he did not address Facebook's decision to move data stored on 1.5 billion non-EU subscribers out of the European Union - specifically from Ireland - so that they would not be affected by the new regulations.
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As he did with a series of questions from United States lawmakers last month, Zuckerberg said he would send follow-up answers later.
The Silicon Valley billionaire managed to dodge numerous questions posed to him during the informal, 90-minute gathering with members of the Parliament's Conference of Presidents in Brussels. He obliquely referred to the question as "a point around how do we separate out security data", adding that "it's very important that we don't have people who aren't Facebook users coming to our service and trying scrape the public data that's available". The CEO agreed to follow up in writing to numerous questions and offered to send a company representative to testify before a future hearing. Belgian Philippe Lamberts said as the meeting was almost over.
It ended with Zuckerberg vowing to get back to his questioners with more specific answers at a later date - before heading to Paris for a meeting with Emmanuel Macron. "And of course, well, you asked for this format for a reason". He also asked how Facebook commercializes that data. But Zuckerberg was invited to respond to the parliamentarians' long list of concerns just seven minutes before the hearing was scheduled to end.
"I look forward to hearing more on plans to help protect our young people and others who are particularly vulnerable to such abuse or trolling". One member of Parliament, Guy Verhofstadt, lamented that Facebook already had apologized for its missteps "fifteen or sixteen times the last decade".
"Are you capable to fix it?" Some sort of regulation is "inevitable.it's about getting it right". He then moved on to equally pointed questions.
"Are you telling the truth, in fact, to us?" he asked the Facebook CEO about the company's pledge to adhere to Europe's privacy laws.
Al Jazeera's Jonah Hull, reporting from Brussels, said that Zuckerberg provided "pretty un-enriching" responses to the MEPs' questions.
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