Facebook is in an arms race with Russia, its CEO told senators as he faced questions about a huge data breach and election meddling.
Mark Zuckerberg appeared before senators in Washington DC after as many as 87m users of the social network had their data used by Cambridge Analytica, a firm working on the Trump 2016 election campaign.
The 33-year-old took the blame for the massive data breach, and said that the company had failed to understand the tools could be used for harm as well as good.
Although he faced little direct questioning about the potential role of Russia in the election meddling, he did claim he was in an "arms race" with the nation.
Mr Zuckerberg said that Facebook believes that it is "entirely possible" that the individuals whose data was improperly obtained by Cambridge Analytica could have been used by the Kremlin-linked troll factory, the Internet Research Agency, to attempt to influence the US Presidential election in 2016.
He said: "There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well.
"So this is an arms race.
"They're going to keep getting better at this, and we need to invest in making sure we keep getting better at this too.
"Which is why we are going to have more than 20,000 people by the end of this year working on security and content review across the company."
He later added: "As long as there are people in Russia whose job it is to interfere with elections around the world, this is going to be an on-going conflict."
Facebook deleted tens of thousands of fake accounts and will investigate tens of thousands of apps that use the platform to identify any other potential breaches.
Mr Zuckerberg frequently admitted to making mistakes, including not informing users as soon as they knew about the data breach.
Senators mostly focused their lines of questioning on privacy, and how Facebook uses and looks after the data it takes from its users.
Sen John Kennedy told Mr Zuckerberg "your user agreement sucks" and Sen Dick Durbin asked if the Facebook CEO would share details of the hotel room in he was staying in.
Mr Zuckerberg answered: "Senator, no, I would probably not choose to share that here."
Sen Durbin said: "I think that maybe is what this is about. Your right to privacy, the limits of your right to privacy, and how much you give away in modern America in the name of quote 'Connecting people around the world'."
Mr Zuckerberg was also challenged about Facebook's claim it was deceived by a university researcher when he sold user data to Cambridge Analytica. Aleksandr Kogan's ability to do so was in fact signed off by Facebook in its contract with him.
Senator Richard Blumenthal showed the terms of service that Mr Kogan provided on a large placard and said "Facebook was on notice that he could sell that information. Have you seen these terms of service before?"
Mr Zuckerberg said he had not seen them, and they were the responsibility of the app review team.
Sen Blumenthal asked: "Has anyone been fired from that app review team?"
Mr Zuckerberg replied that no one had been dismissed.
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