Facebook founder Mark Zuckerberg has told US senators that the social media giant is in an arms race with Russia.
The 33-year-old CEO appeared before lawmakers in Washington DC after as many as 87m users of the social network had their data harvested by Cambridge Analytica, a firm working on the Trump 2016 election campaign.
Although he faced little direct questioning about the potential Russia election meddling, he did claim he was in an "arms race" with the nation.
He admitted it is "entirely possible" that the data improperly obtained by Cambridge Analytica could have been used by the Kremlin-linked troll factory - the Internet Research Agency – in an attempt to influence the US Presidential election in 2016.
"There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well,” he said.
"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 ongoing conflict."
The Facebook CEO said his company will investigate "tens of thousands" of apps to discover if any other companies have accessed data in a similar way to Cambridge Analytica (CA).
The social network says it is in the process of letting up to 87 million users know that their information may have been accessed by CA, and in a packed room on Capitol Hill, Mr Zuckerberg repeated his admission that the company "didn't do enough" to stop its tools "being used for harm".
After a torrid few weeks for the company's share price investors rewarded Mr Zuckerberg for appearing on the Hill.
Facebook stocks rose 4.5% on Tuesday, which is said to have swelled the entrepreneur's fortune by several billion dollars.
Speaking to the Senate Judiciary and Commerce Committees, the tech entrepreneur apologised that Facebook had not taken a "broad enough view" of its responsibility for people's public information.
"It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he said, in words from a prepared statement.
Mr Zuckerberg said its audit of third-party apps would highlight any misuse of personal information, and said the company would alert users instantly if it "found anything suspicious".
When asked why the company did not immediately alert the 87 million users whose data may have been accessed by Cambridge Analytica (CA) when first told about the "improper" usage in 2015, Mr Zuckerberg said Facebook considered it a "closed case" after CA said it had deleted it.
"In retrospect it was clearly a mistake to believe them," he said.