(CNN Money) -- Mark Zuckerberg has apologized many times in his years running Facebook, but this time is different. At least that was the message he delivered during his debut appearance on Capitol Hill.
The Facebook CEO formally apologized to Congress on Tuesday for mistakes that led to the Cambridge Analytica scandal and stressed that his company is rethinking its responsibilities to users and society.
"It's clear now that we didn't do enough to prevent these tools from being used for harm as well. That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy," Zuckerberg said in opening remarks before a joint hearing of the Senate Judiciary and Commerce committees.
"We didn't take a broad enough view of our responsibility and that was a big mistake," he continued. "It was a big mistake. And I'm sorry. I started Facebook. I run it and I'm responsible for what happens here. "
When pressed on why people should trust him now, after years of his promises to do better, Zuckerberg admitted that "we have made a lot of mistakes in running the company."
"It's pretty much impossible to start a company in your dorm room and grow it to the scale we are at now without making some mistakes," he said. Now, however, "I would say we are going through a broader philosophical shift in how we run the company."
For most of its history, Facebook focused on "building tools," he said. Now, Facebook recognizes the need to "take a more proactive role."
The appearance marks the first time that Zuckerberg has testified before Congress. Zuckerberg, 33, swapped his usual gray t-shirt and jeans attire for a dark blue suit and light blue tie.
He appeared somber as he walked in to testify, and spoke before a packed room, with 44 senators in attendance. The number of senators, and the time constraints for each, limited the potential for followup questions to and grilling of the CEO.
"It's extraordinary to hold a joint committee hearing. It's even more extraordinary to have a single CEO testify before nearly half the United States Senate," said Sen. John Thune, chairman of the Commerce Committee. "Then again, Facebook is extraordinary."
Zuckerberg will testify again on Wednesday before the House Energy and Commerce Committee.
The Congressional hearings come nearly a month after news broke that Cambridge Analytica, a data firm with ties to President Donald Trump's campaign, accessed information from as many as 87 million Facebook users without their knowledge.
The data scandal wiped away tens of billions of dollars from Facebook's market value, prompted political scrutiny on both sides of the Atlantic and even raised the once unthinkable question of whether Zuckerberg should step down as CEO. It also reignited long simmering concerns about Facebook's impact on the world's privacy, civil discourse and domestic institutions.
While Zuckerberg took responsibility for Facebook's shortcomings in preventing abuse of the platform, he nonetheless continued to defend the company's core business model: Using personal data to target ads.
"We think offering an ad-supported service is the most aligned with our mission to connect everyone in the world. We want to offer a free service that everyone can afford," he said.
In the days leading up to the hearings, Facebook released a torrent of product and policy updates to address concerns about data privacy and election meddling. Zuckerberg, once press shy, also launched an apology tour as part of a rare media blitz.
Behind the scenes, Zuckerberg and his team did mock hearings over the past week in a conference room at Facebook set up to look like a congressional hearing room. Zuckerberg plans to be contrite in his appearances before lawmakers. He will make the case for Facebook — why it helps people's lives — but be ready to push back when appropriate.
"He's nervous, but he's really confident," the source said. "He's a smart guy."
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