Here are some of the topics that the committee members are likely to ask about:
Legislation to regulate the use of private data
On Tuesday, several senators said Facebook couldn’t be trusted to regulate itself.
“Most Americans have no idea what they are signing up for because Facebook’s terms of service are beyond comprehension,” Mr. Graham said in a statement after the hearing.
He called Facebook a “virtual monopoly” and said “continued self-regulation is not the right answer when it comes to dealing with the abuses we have seen on Facebook.”
Three senators introduced privacy legislation on Tuesday that would require users’ permission to collect and share their data.
Mr. Zuckerberg repeatedly said he was open to regulations but that it would have to be the “right” regulations with the right details. On Wednesday, Mr. Zuckerberg will most likely be asked specifically to agree to privacy legislation that requires permission for data collection.
Cambridge Analytica and Russia’s election interference
Senators pressed Mr. Zuckerberg on why Facebook didn’t inform users about the harvesting of user data by Cambridge Analytica, a political consulting firm with ties to the Trump campaign, in 2015, when it was informed of the data abuse.
Mr. Zuckerberg did not admit that the company explicitly decided to withhold that information from consumers, but he said the company made a mistake in not telling users.
Expect more questions on the Cambridge Analytica episode that could inform the Federal Trade Commission’s investigation of the episode as well as provide Democrats with more ammunition about Russia’s meddling in the 2016 election.
Partisan bias and Facebook’s responsibility as a publisher
Mr. Zuckerberg will most likely confront more questions about how the company can determine what type of content should be published on its site but not define itself as a media company.
The proliferation of so-called fake news has put Mr. Zuckerberg in an awkward spot, as the company promises to do a better job of weeding out propaganda and falsehoods but insists it cannot police free speech.
At Tuesday’s hearing, Senator Ted Cruz, Republican of Texas, grilled Mr. Zuckerberg on allegations that Facebook had censored content from Trump supporters and conservatives.
And Democrats expressed concerns about the inflammatory stories that were published on the platform by the Internet Research Agency, a private company with Kremlin ties.
How Facebook Works
The Senate hearing made clear that lawmakers aren’t quite sure what Facebook’s business model is or how it works, including what the difference is between selling user data to advertisers and allowing advertisers to target ads to an aggregated slice of Facebook users.
The technological gap between Silicon Valley and Washington was apparent when Senator Roger Wicker, a Republican of Mississippi, asked about internet regulation.
Mr. Zuckerberg explained that when thinking about regulations, government officials need to differentiate between internet companies like his and broadband providers, the companies that build and run the “pipes” that carry internet traffic, like AT&T and Comcast.
The difference is at the heart of net neutrality, a hotly debated regulation that was overturned last year. The rules prevent internet service providers from favoring the flow of all internet content through their pipes.
“I think in general the expectations that people have of the pipes are somewhat different from the platforms,” Mr. Zuckerberg said.
“When you say pipes, you mean?” Mr. Wicker asked.