At one point during the Tuesday hearing, for example, Senator Lindsey Graham of South Carolina asked Mr. Zuckerberg about his company’s top competitors, singling out one in particular.
“Is Twitter the same as what you do?” Mr. Graham asked.
Mr. Zuckerberg did acknowledge some similarities between the two social networking sites. But Mr. Graham — or, at least, someone in his office — already appeared to have some fluency in both platforms. After Tuesday’s hearing, Mr. Graham posted his statement on the hearings to Facebook and uploaded a screenshot of his statement to Twitter.
Similarly, early in the hearing, Senator Orrin Hatch of Utah asked Mr. Zuckerberg, “How do you sustain a business model in which users don’t pay for your service?”
“Senator, we run ads,” Mr. Zuckerberg responded, providing a basic fact that’s central to Facebook’s operations.
Though many of the questions the 44 senators asked Mr. Zuckerberg suggested some confusion over how his company operates, all of those senators still use his platform as an important communication tool.
Facebook, on a section of its website addressing politicians and government institutions, says its tools “help increase two-way conversations between government officials and the people they represent.” It encourages elected officials to build a following by sharing behind-the-scenes content and engaging with fans and constituents.
The senators who questioned Mr. Zuckerberg on Tuesday appeared to take that advice to heart. Twenty-five of them used their Facebook pages on Tuesday to comment on the hearing.
Those active Facebook presences might seem at odds with the concerns the senators raised over the site’s business practices and the way Facebook treats its users. But surveys released by the Pew Research Center suggest the senators’ views on the platform are similar to those of the American public.
About two-thirds of adults in the United States use Facebook, according to a Pew survey conducted in January 2018, and 74 percent of those users said they visit the site daily.
Yet that heavy usage didn’t breed comfort with Facebook’s data policies. In a 2014 survey, about 80 percent of Americans said they didn’t feel secure using social media to share private information. And a survey released in 2017 found that close to half of Americans didn’t trust social networking sites like Facebook to protect their information.
Despite those attitudes, Facebook remains widely adopted in the United States, and the gulf between opinions about the site and the behavior of its users has led some to posit that Facebook may be too ubiquitous for users to quit. And the online behavior of senators on Tuesday suggested that they may be as vulnerable to that problem as the constituents they serve.
Mr. Cruz, among the most active senators on Facebook while Mr. Zuckerberg was in Washington, pressed the Facebook executive on the site’s treatment of conservative news media and politics. Of the senator’s nine posts on Tuesday, four were tied to Mr. Zuckerberg’s testimony.
Despite Mr. Cruz’s concerns, he and his office have no plans to change their approach to the platform, according to Catherine Frazier, a spokeswoman for the senator.
“We use Facebook because it is supposed to be an unvarnished, unfiltered way to engage with our constituents,” she said.
Like Mr. Cruz, Senator Kamala Harris, Democrat of California, expressed deep reservations about Facebook when she questioned Mr. Zuckerberg.
“I’m concerned about how much Facebook values trust and transparency,” she said. She then criticized what she said was his inability to answer questions about what data Facebook stores and whether the site can track its users’ activity.
She later reiterated her skepticism over Facebook’s commitment to transparency in a Facebook post after the hearing.
“Most of the people we’re reaching on Facebook are the people that we want to inform about the privacy concerns that Ms. Harris raised during the hearing,” said Tyrone Gayle, a spokesman for Ms. Harris.
“She wants to make sure her constituents have this information at their disposal,” he added.
On Wednesday, it was the turn of House members to question Mr. Zuckerberg. By some accounts, they seemed a bit more knowledgeable about the platform and asked tougher questions overall.
[Listen here: Excerpts from the hearing, with analysis from Kevin Roose, The Times technology columnist, on “The Daily” podcast.]
Representative Kathy Castor, Democrat of Florida, drew attention for sharp questions about privacy that left Mr. Zuckerberg reeling. Afterward, she shared a story about her line of attack — on Facebook, of course.