After saying Facebook's policy is not to "fact-check politicians' speech", Zuckerberg explained the reasoning.
"The reason for that is that we believe that in a democracy it is important that people can see for themselves what politicians are saying," Zuckerberg said.
Facebook ran a political ad which falsely accused former Vice President Joe Biden of blackmailing Ukrainian officials to stop an investigation of his son.
Last week, the Biden campaign sent a letter to Facebook arguing the ad should be taken down.
The Facebook CEO last appeared before Congress in April 2018 when he fielded 10 hours of questions over two days from House and Senate panels on political consulting firm Cambridge Analytica's misuse of Facebook customer data to interfere in the U.S. presidential election.