Lawmakers also put spotlights on problems with tech companies operating platforms relied upon by rivals.
Sundar Pichai was pressed regarding whether Google's ad platform tactics hurt other internet players such as news outlets.
Apple's Cook faced tough questioning over the market power of the company's App Store and its treatment of developers.
"We treat all app developers the same," Tim Cook said. "We do not retaliate or bully people."
Jeff Bezos, in his first appearance before a congressional committee, defended Amazon's dealings with third-party sellers after a blistering attack from Cicilline.
"We've heard from third-party sellers again and again during the course of our investigation that Amazon is the only game in town," the committee chair said to the company founder.
Jeff Bezos disputed the characterization while adding, "There are a lot of options for small firms.. I think we are the best one."
Playing Politics
President Donald Trump, who has accused Facebook and Twitter of censoring his remarks and being biased against conservatives, weighed in with a tweet shortly before the hearing.
"If Congress doesn't bring fairness to Big Tech, which they should have done years ago, I will do it myself with Executive Orders," Trump said.
Representative Jim Jordan, a Republican from Ohio, said during the hearing: "I'll just cut to the chase -- Big Tech is out to get conservatives. That's not a suspicion. That's not a hunch. That's a fact."
Democrat Jamie Raskin of Maryland rejected the argument, saying Facebook and other platforms have failed to contain misinformation from Trump and his supporters, including unverified Covid-19 claims.
"If Facebook is out there trying to repress conservative speech they're doing a terrible job," Jamie Raskin said. "I don't understand this endless whining" from Republicans.
Moving Forward
Some analysts said the hearing could set the stage for revising US antitrust laws, which at present make it difficult for enforcers to target companies simply for being big.