Facebook and Twitter grilled over US election actions
Mark Zuckerberg and Jack Dorsey face questions about misinformation labels and account bans.
The two were summoned to answer questions about how their platforms had limited distribution of a controversial article about Joe Biden's son published ahead of the US election.
But they are also being challenged over their handling of posts by President Trump and others who have contested the vote's result.
The tech firms face new regulations.
In particular, President-elect Biden has suggested that protections they currently enjoy under a law known as Section 230 should be "revoked".
It says the platforms are generally not responsible for illegal or offensive things users post on them.
Mr Biden has said this allows them to spread "falsehoods they know to be false".
Republicans have also voiced concern about the law. They claim it lets social media companies take decisions about what to leave up and take down without being transparent about why, making bias possible.
"When you have companies that have the power of governments, have more power than traditional media outlets, something has to give," said the Republican Senator Lindsey Graham, who is chairman of the Senate Judiciary Committee.