Facebook and Twitter have said that they took too long to tackle foreign campaigns to meddle in US elections.
Responding to lawmakers, Facebook's chief operating officer Sheryl Sandberg said the social network was "too slow" to act on election interference.
Twitter's chief executive Jack Dorsey said his platform was "unprepared and ill-equipped" for the "weaponisation" of debate.
Google did not show up to the Senate Intelligence Committee hearing.
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 focused on what the technology giants were doing to prevent future election meddling.
It followed 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.
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