Facebook CEO Mark Zuckerberg has told US senators his company is in a constant battle with Russian operators seeking to exploit the social network.
"This is an arms race. They're going to keep getting better," he said.
Mr Zuckerberg was answering questions in the wake of the Cambridge Analytica data collection scandal.
He also revealed Robert Mueller, the special counsel investigating alleged Russian meddling in the 2016 election, had interviewed Facebook staff.
Zuckerberg says that Facebook has been served with subpoenas by special counsel Mueller's office - but then backs off and says that Facebook is "working with them", including meetings.— Anthony Zurcher (@awzurcher) April 10, 2018
Mr Zuckerberg said he has not been among those interviewed by Mr Mueller's office.
But he added: "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."
In February, Mr Mueller's office charged 13 Russians with interference in the 2016 election, along with three Russian companies.
One was the Internet Research Agency, sometimes referred to as a "Russian troll farm", which the indictment said had a "strategic goal to sow discord in the US political system".
Mr Zuckerberg said the company was now developing new tools to identify fake accounts.
"There are people in Russia whose job it is to try to exploit our systems and other internet systems and other systems as well. We need to invest in getting better at this too."
The Facebook chief fended off questions from senators about how the social network might be regulated more closely.
Senator John Kennedy warned him: "I don't want to have to vote to regulate Facebook. But by God, I will. That depends on you... Your user agreement sucks."
When pressed, the 33-year-old billionaire tech titan said he would welcome regulation, if it was the "right regulation," though he avoided specifics.
He was appearing in front of a joint session of several US senate committees, after it was revealed in recent weeks that about 87 million people had their profile information accessed by marketing firm Cambridge Analytica.
During the hearing, Mr Zuckerberg also said:
- "It's clear now that we didn't do enough to prevent these tools from being used for harm"
- "In retrospect it was clearly a mistake" to believe Cambridge Analytica deleted data, without further examination
- He does not "feel like" Facebook has a monopoly
- That there would always be a free version of Facebook, leaving open the possibility of a paid, ad-free version of the social network
- Dealing with hate speech automatically has "a higher error rate than I am happy with"
- He was personally concerned about the possibility of political bias at the company
By the first break in proceedings, Facebook's share price had risen by almost 5%, as markets reacted favourably to Mr Zuckerberg's performance, increasing his net worth by an estimated $3bn.
What is the Cambridge Analytica scandal about?
The company is best known for its association with Donald Trump's 2016 election campaign, and has claimed it played "an integral part" in Mr Trump's victory.
But it says none of the data involved in the recent scandal was used to help the Trump campaign.
The news that a personality quiz developed by an academic, Aleksandr Kogan, had collected data not just from the people who used it but also from their Facebook friends was revealed by newspaper investigations. Mr Kogan then sold the data to Cambridge Analytica.
Both companies lay the blame on Mr Kogan. Facebook says that although users gave permission to Mr Kogan's app to collect their information, selling it on was against the terms of service.
Cambridge Analytica, meanwhile, claims it did not know the information had been obtained improperly. During Mr Zuckerberg's appearance, the firm tweeted it was "advising" news media - through its lawyers - about coverage.
We have instructed our lawyers to send letters to news media who have been covering this story, advising them not to repeat false and unfounded allegations as fact.— Cambridge Analytica (@CamAnalytica) April 10, 2018
Both companies also say they moved to have the data deleted once they learned of the problem in 2015.
Hours ahead of the congressional hearing, Facebook also revealed that private messages from some 1,500 users were included in the data collection.