Democrats argued the refusal of Facebook to fact check political ads will make into a hotbed of misleading and false attacks, which could skew future elections.
“Could I run ads targeting Republicans in primaries saying that they voted for the Green New Deal?” asked Rep. Alexandria Ocasio-Cortez (D-NY), pressing Zuckerberg to see where he would draw the line on false advertising on Facebook.
“I don't know the answer to that off the top of my head,” Zuckerberg said, as Ocasio-Cortez pressed him for an answer.
“So you don't know if I will be able to do that,” Ocasio-Cortez continued.
“I think, probably,” Zuckerberg answered.
While Democrats complained about a lack of fact checking on political ads, most Republicans said there should be no limits on the freedom of speech through Facebook.
"I don’t want you to be bullied by politicians who want to censor politically incorrect speech," Rep. Andy Barr (R-KY) told Zuckerberg at one point.
At the hearing - which stretched over six hours - Zuckerberg also took flak again over Facebook's plan to develop a new cryptocurrency, known as Libra.
As in previous appearances before Congress, Zuckerberg said his company will go ahead with the Libra cryptocurrency plan only after U.S. regulators give it a green light.
So far, that has not happened.
"Is it a currency? Are you a bank?" asked Rep. Ed Perlmutter (D-CO), as both parties questioned why Zuckerberg was going to base his Libra currency in Switzerland - and not in the United States.
"Do you consider Libra to be money? Perlmutter asked. "I consider Libra to be a payment system," Zuckerberg replied, as lawmakers expressed concern that it could be used for money laundering and by terrorist groups.
"We have to regulate this," Perlmutter concluded.