Executives from Facebook and Twitter faced threats of legislative action from sceptical US politicians today over what many members of Congress see as a failure to combat foreign efforts to influence US politics.
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'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 US election, but insisted it is doing better.
"We've removed hundreds of pages and accounts involved in coordinated inauthentic behaviour - meaning they misled others about who they were and what they were doing," Ms Sandberg told the Senate Intelligence Committee.
"When bad actors try to use our site, we will block them," she said.
Mr 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% and Facebook around 1.2% lower. Shares of Alphabet sank 1.4%.
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.