Shares of social media companies fell after the start of the hearing when Senator Mark Warner, the committee's Democratic vice chairman, said the companies and Alphabet Inc's Google have not done enough to slow the spread of disinformation, and threatened "action."
Facebook Chief Operating Officer Sheryl Sandberg, who testified alongside Twitter Chief Executive Jack Dorsey, acknowledged that the company was too slow to respond to Russian efforts to interfere in the 2016 U.S. election, but insisted it is doing better.
"We've removed hundreds of pages and accounts involved in coordinated inauthentic behavior - meaning they misled others about who they were and what they were doing," Sandberg told the Senate Intelligence Committee.
"When bad actors try to use our site, we will block them," she said.
Dorsey, sporting a straggly beard, nose ring and open-collared shirt, insisted Twitter's monitoring has tightened, including notifying law enforcement last month of accounts that appeared to be located in Iran.
The committee also asked Google to testify, but declined an offer to hear from Chief Legal Officer Kent Walker rather than Alphabet Chief Executive Larry Page.
Facebook, Twitter and other technology firms have been on the defensive for many months over political influence activity on their sites as well as concerns over user privacy.
Their stocks fell as the hearing progressed, with Twitter down 4.5 percent and Facebook around 1.2 percent lower. Shares of Alphabet sank 1.4 percent.
Investors are concerned "about how government and policies could pose as a threat to these growth stocks," said Ryan Nauman, market strategist at Informa Financial Intelligence in Zephyr Cove, Nevada.
Senator Richard Burr, the committee's Republican chairman, said, "Unfortunately, what I described as a 'national security vulnerability,' and 'unacceptable risk,' back in November remains unaddressed."
"Clearly, this problem is not going away. I'm not even sure it's trending in the right direction," he added.
Warner said social media companies were doing better at combating disinformation, but their efforts were insufficient.
"I'm skeptical that, ultimately, you'll be able to truly address this challenge on your own. Congress is going to have to take action here," Warner said.
Republican Senator Susan Collins said she had learned from a university report that she had been targeted some 270 times by Russian-linked trolls on Twitter, asking Dorsey why the company does not tell users when they have been attacked.Dorsey said it was "unacceptable" that she was not told.
Before the hearing, President Donald Trump, without offering evidence, accused the companies themselves of interfering in the upcoming US mid-term elections, telling the Daily Caller that social media firms are "super liberal."
Trump told the conservative outlet in an interview conducted on Tuesday that "I think they already have" interfered in the Nov. 6 election. The report gave no other details.
Executives from the companies, which have repeatedly denied political bias, have traveled to Washington several times to testify in Congress, including 10 hours of questioning of Facebook Chief Executive Mark Zuckerberg over two days in April.
Senate Intelligence has been looking into Russian efforts to influence US public opinion throughout Trump's presidency, after US intelligence agencies concluded that Kremlin-backed entities sought to boost his chances of winning the White House in 2016.
Moscow denies involvement, and Trump - backed by some of his fellow Republicans in Congress - has repeatedly dismissed investigations of the issue as a partisan witch hunt or hoax.
REPUBLICANS CHARGE BIAS
Some Republicans have also charged social media companies with bias against Trump and other conservatives.
Conspiracy theorist Alex Jones held a press conference outside the Senate hearing room as the testimony began to criticize the social media companies for banning him and his website Infowars from their platforms.
Twitter's Dorsey was to follow his Senate testimony with an appearance at an afternoon hearing looking at that issue in the House of Representatives.
Trump faulted Twitter on July 26, without citing any evidence, for limiting the visibility of prominent Republicans through a practice known as shadow banning.
In the Senate, both Burr and Warner pressed the social media companies to do more.
Many senators expressed anger at Google, which was represented in the hearing room by an empty chair next to Sandberg. Republican Senator Marco Rubio said the company might have skipped the hearing because it was "arrogant."
Google did release written "testimony" from Walker ahead of the hearing, even though he was not expected to appear. Walker said Google takes foreign interference in politics very seriously.
A committee spokesperson said Walker's "commentary" was not testimony, adding, "We wish his enthusiasm for participating in the company's public hearing extended to his company's senior leadership, and that they were willing to answer the committee's questions."