Zuckerberg explained that the company has taken steps to make ads transparent and anyone who wants to run a political or social issue ad has to go through its authorisation process first.
Democrats focused their criticism mainly on hate speech, misinformation and other content that can incite violence, keep people from voting or spread falsehoods about the coronavirus.
The CEOs were to testify on Section 230 of the Communications Decency Act, which protects tech companies from liability over the content posted by users on their online services.
"The truth is this work is incredibly important but it's done completely wrong and while the policy is constantly changed the situation seems to get worse."
Both Facebook and Twitter faced backlash from the Republicans and US President Donald Trump for their move to block and censor the article that appeared in The New York Post and was critical of Joe Biden.
The CEOs will testify on Section 230 of the Communications Decency Act, which protects tech companies from liability over the content posted by users on their online services, reports CNBC.
Kevin Systrom and Mike Krieger wanted a quick way to share photos in an age when smartphones cameras had people capturing all kinds of moments in pictures.
