Facebook chief executive Mark Zuckerberg has told the US Congress in written testimony that the social media network did not do enough to prevent misuse of data and apologised.
"It's clear now that we didn't do enough to prevent these tools from being used for harm..." he said in testimony released by the US House Energy and Commerce Committee on Monday. " That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy."
Mr Zuckerberg is scheduled to appear before two congressional committees on Tuesday and Wednesday, to answer questions about Facebook's admission that the personal information of up to 87 million users, mostly in the US, may have been improperly shared with political consultancy Cambridge Analytica.
"We didn't take a broad enough view of our responsibility, and that was a big mistake," his testimony continued. "It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here."
Facebook said on Sunday the company plans to begin telling affected users on Monday. The company's data practices are under investigation by the US Federal Trade Commission.
Mr Zuckerberg also said Facebook's major investments in security "will significantly impact our profitability going forward". Facebook shares were up 1.7 percent in midday trading.
London-based Cambridge Analytica, which counts US President Donald Trump's 2016 campaign among its past clients, has disputed Facebook's estimate of the number of affected users.
On Friday, Facebook backed proposed legislation requiring social media sites to disclose the identities of buyers of online political campaign ads and introduced a new verification process for people buying "issue" ads, which do not endorse any candidate but have been used to exploit divisive subjects such as gun laws or racism.
The steps are designed to deter online information warfare and election meddling that US authorities have accused Russia of pursuing, Mr Zuckerberg said on Friday. Moscow has denied the allegations.
Mr Zuckerberg's testimony said the company was "too slow to spot and respond to Russian interference, and we're working hard to get better".
He vowed to make improvements, adding it would take time, but said he was "committed to getting it right".