Facebook and Twitter are being questioned by US senators, to find out how they will prevent meddling in upcoming elections.
Facebook is being represented by its chief operating officer Sheryl Sandberg, while Twitter sent chief executive Jack Dorsey.
As anticipated, Google did not show up.
Opening the hearing, Democratic senator Mark Warner said he was “deeply disappointed” that Google “chose not to send its own top corporate leadership”.
The senate committee is focusing on what the technology giants are doing to prevent future election meddling.
The hearing follows claims that Russia and other foreign actors spread misinformation and propaganda ahead of the 2016 presidential election.
“With the benefit of hindsight, it is obvious that serious mistakes were made by both Facebook and Twitter. You, like the US government, were caught flat-footed by the brazen attacks on our election. Even after the election, you were reluctant to admit there was a problem,” said Mr Warner.
He warned that the social networks could face new regulations.
“The era of the wild west in social media is coming to an end. Where we go from here is an open question,” he told the hearing.
Is Twitter censoring conservative voices?
Twitter’s Dorsey will also face questions from the House Committee on Energy and Commerce about perceived conservative censorship on its platform.
President Trump has repeatedly accused Google, Facebook and Twitter of political bias and threatened regulation of the platforms.
In written testimony submitted ahead of the hearing, Mr Dorsey denied that his firm de-emphasised certain accounts in search results, a practice known as “shadow banning”.
“Twitter does not use political ideology to make any decisions, whether related to ranking content on our service or how we enforce our rules,” he said.
“We do not shadow-ban anyone based on political ideology.”