Republican members of the committee attacked the power that social media companies have to moderate content on their platforms, accusing them of making politically inclined phone calls while hiding behind a decades-old liability shield.
"I don't want the government to take on the job of telling America which tweets are legitimate and which are not," said panel chairman Senator Lindsey Graham of South Carolina. "But when you have companies that have government power, that have far more power than traditional media, something has to give."
President Trump and his allies have attacked the Silicon Valley platforms for years because they are allegedly biased against conservatives. You have pointed out the liberal policies of corporate employees and incidents of moderation involving Republicans or conservative media. Your evidence for these claims has always been anecdotal, and many right-wing figures have found huge followers online.
Mr. Zuckerberg and Mr. Dorsey said that while their companies sometimes made mistakes, their policies are fair and serve the best interests of their users.
Republicans spent much of their time focusing on individual business decisions. Mr Graham took an exception to the way Twitter and Facebook had initially narrowed the reach of a New York Post article about Hunter Biden, son of President-elect Joseph R. Biden Jr. The article prompted the committee to require the directors of the two companies to testify.
"It seems to me like you are the ultimate editor," said Mr. Graham.
Her comments reflected the way Conservatives are increasingly attacking corporations for coping with a fragile post-presidential election period when President Trump refused to admit despite significant leadership from Mr Biden.
Mr Graham questioned Twitter's decision to flag a post by a Republican politician as a "controversial" allegation of electoral fraud. Senator Mike Lee, a Republican from Utah, said one of his Facebook posts about the election was flagged by the platform.
"Well, maybe such concerns are no longer mainstream in Palo Alto," Lee said, referring to the town in Silicon Valley, not far from Facebook. "But in the rest of America they're not out of the mainstream."