Mark Zuckerberg acknowledged today that Facebook didn’t do enough to prevent the powerful tools it created to connect 2 billion people from being used in harmful ways. The remarks came before a joint session of the Senate Judiciary and Commerce Committees, one of two planned hearings for Zuckerberg this week.

“We didn’t take a broad enough view of our responsibility,” the CEO said in his opening remarks. “It was a big mistake. It was my mistake.”

Capitol Hill legislators called on Zuckerberg to answer questions about privacy raised by the Cambridge Analytica data breach and address how the social network plans to protect its users from such abuses in the future.

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The Facebook co-founder doffed his trademark hoodies in favor of a Washington, D.C.-friendly suit and tie for his first of two appearances before Congress this week. Today’s session began shortly after 2:15 PM ET and was approaching the five-hour mark late this evening.

Zuckerberg today again laid out how Facebook has responded to the damaging disclosures, including attempting to get to the bottom of what Cambridge Analytica did and a full investigation of “every single third-party app” that had access to large amounts of information to determine if there are other instances of developers gaining improper access to user data.

Sen. John Thune (R-SD), chairman of the Senate Commerce Committee, sought to frame the discussion as narrowly focused on privacy and the information Facebook users share while using the popular social network, while avoiding the politically charged role of the data analytics company Cambridge Analytica, which advised Donald Tump’s 2016 presidential campaign.

“Mr. Zuckerberg, the company you created, your story, represents the American dream,” Thune said. “Many are inspired by what you have done. You have an obligation, and it’s up to you, to ensure that dream doesn’t become a privacy nightmare for the scores of people who use Facebook.”

But Democrats didn’t heed the call to avoid the political ramifications of the Cambridge Analytica disclosures, in which the data of as many as 87 million Facebook users appeared to have been harvested, and sold, through a third-party app.

Sen. Dianne Feinstein, the California Democrat who is facing a primary challenge in her re-election bid, expressed concern about Russian agents exploiting the Facebook platform in a coordinated misinformation campaign seen by 157 million Americans, and Cambridge Analytica’s use of data to create “psychological warfare tool” to influence the U.S. election.

Zuckerberg sought to assure lawmakers worried about Russian meddling during the upcoming midterm election that Facebook is taking steps to identify and close fake accounts like those used in the past to circulate “inauthentic” information.

“One of my greatest regrets is we were too slow in identifying Russian activity” in the 2016 elections, Zuckerberg said, adding that “Our top priority is protecting the integrity of elections around the world.”

Zuckerberg confirmed, under questioning from Sen. Patrick Leah (D-VT), that Facebook is “working with” special counsel Robert Mueller in his probe of Russian interference in the election, though he has not personally been interviewed by Mueller’s team.

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“I want to be careful here, because that — our work with the special counsel is confidential, and I want to make sure that, in an open session, I’m not revealing something that’s confidential,” Zuckerberg said.

Earlier this year, Mueller indicted 13 Russians and three companies in an indicted that spelled out a sophisticated campaign to subvert the 2016 presidential election, through an online propaganda campaign that included the purchase of ads on Facebook.

Sen. Lindsey Graham (R-SC) wondered aloud if consumers who were upset with Facebook’s privacy practices had any alternative, much as car buyers who grow unhappy with their Ford might opt for a Chevy instead. He asked Zuckerberg, point blank, if Facebook is a monopoly.

“It certainly doesn’t feel like it,” Zuckerberg responded, adding that the social network has competitors in multiple areas.

Sen. Ted Cruz (R-TX) aired familiar criticism that Facebook is biased against conservatives. He cited a 2016 Gizmodo report of Facebook curators allegedly suppressing conservative viewpoints in the trending news section of the site, and Facebook’s decision, just this month, that videos posted by a pair of Trump supporters, sisters who refer to themselves as Diamond and Silk, were dangerous.

“To many Americans, that appears to be a pervasive pattern of bias,” Cruz said.

Zuckerberg said he understood the root of such concerns — “Silicon Valley is an extremely left-leaning place” — though he said the social network is a non-ideological forum for the exchange of ideas.

“I’m very committed to making sure Facebook is a platform for all ideas,” Zuckerberg said. “That is a founding principle.”

Sen. Chuck Grassley, the Iowa Republican who chairs the Senate Judiciary Committee, pressed Zuckerberg on the “massive amount of data” it collects about users, noting that Facebook emphasizes benign uses of this information, such as connecting with friends. He asked why the social network wasn’t more forthcoming in disclosing how data is used.

“This gets to an issue we in the technology industry find challenging,” Zuckerberg said. People don’t read lengthy terms of use agreements, he said. So Facebook tries to give people tools to control who sees a particular photo or post.

Sen. Bill Nelson, a Florida Democrat who serves as ranking member of the Senate Commerce Committee, said he wonders if users are truly aware of how advertisers use the information they volunteer on Facebook for targeting purposes. He offered the example of mentioning “a certain type of chocolate” he loves — and subsequently receiving ads for the product.

Zuckerberg said users can control what personal details are shared with third parties, who use the information to deliver more relevant advertising.

“Even though some people do not like ads, people do not want ads that are irrelevant,” Zuckerberg said. “The overwhelming feedback we get from our community people would rather have relevant content than not.”

Zuckerberg said Facebook opted for an ad-supported business model to accomplish its broader goal of connecting billions of people on the planet — including those who couldn’t afford to pay. Although he appeared to hint at a premium version of the social network in testimony, saying,” We don’t offer an option today to pay to not show ads.”

Wall Street appears to have been impressed with Zuckerberg’s testimony. The company’s stock, which has been battered by the Cambridge Analytica disclosures, closed up 4.5% at $165.04 today.