In prepared remarks released by a USA congressional panel, Zuckerberg admitted he was too idealistic and failed to grasp how the platform - used by two billion people - could be abused and manipulated.
Zuckerberg said one of his "greatest regrets" was that Facebook was "slow in identifying the Russian information operations" during the USA presidential campaign in 2016.
Zuckerberg said "there will always be a version of Facebook that is free" and "we don't offer an option today for people to pay to not show ads". Your right to privacy, the limits of your right to privacy, and how much you'd give away, in modern America, in the name of "connecting people around the world".
"It's clear now that we didn't do enough to prevent these tools from being used for harm".
Perhaps the only time Zuckerberg truly seemed to be under pressure was when Harris pressed him to explain why Facebook did not disclose the Cambridge Analytica matter to the public in 2015, and why it waited until it was forced to do so by media reports this year.
His apology came as Facebook faced a widening scandal where a British political consultancy firm Cambridge Analytica improperly gathered detailed information on 87 million of its users.
It made people think about just how lucky some of them are. "We've updated our policy to make sure we don't make that mistake again".
The US Federal Trade Commission is investigating whether Facebook violated an agreement it signed with the agency in 2011 by its actions in the Cambridge Analytica scandal. VIDEO: Sen. Dianne Feinstein questions Facebook CEO Mark ZuckerbergHe kept eye contact with the senators, not appearing to rely on notes or a script. California Senator Kamala Harris wanted to know if Zuckerberg was involved in not informing users that their information had been compromised. He only spoke of the "conversation in company at the time", and the ultimate outcome, which was to ban Cambridge Analytica from the platform.
Facebook CEO Mark Zuckerberg testifies before a joint hearing of the Senate Committee on the Judiciary and the Senate Committee on Commerce, Science, and Transportation in Washington on April 10, 2018. You know that and I know that.
When asked if his employees had been interviewed, he also responded yes, but added, "I have not". Dan Sullivan, R-Alaska, that "I'm not the type of person who thinks that all regulation is bad". It speaks to the responsibility that Facebook has historically felt regarding policing content on its platform - or, more accurately, the lack thereof.
On Monday, Zuckerberg ditched his trademark T-shirt for a sombre dark suit and tie as he made the rounds on Capitol Hill and sounded contrite about Facebook's conduct.
But he said he tries to make sure Facebook does not have any bias in the work that it does.