Swapping his trademark t-shirt for a suit and tie, Mark Zuckerberg sat in the hot seat Tuesday afternoon as senators fired off questions about how he plans to address Facebook's growing list of issues — from preventing election meddling to protecting users' privacy.
The testimony before the Senate Judiciary and Commerce Committees marked Zuckerberg's first time in front of Congress. The notoriously private billionaire took questions from the panel of senators, each one taking a five-minute turn to ask him anything.
The process yielded some new insights — along with a few awkward moments.
Here's what you need to know so far. Zuckerberg's testimony will continue Wednesday when he faces the House Energy and Commerce Committees.
Someone at Facebook has a lot of homework to do
When Zuckerberg didn't have a specific answer to a question, he punted to a familiar response: "I'll have someone on my team get back to you."
Whether that follow-up between the committee and Facebook actually happens will remain to be seen. However, it's safe to say that someone at Facebook may have a lot of homework coming their way.
The refrain became so common that it led people on Twitter to suggest it was time to take a drink every time Zuckerberg said it. And with the hearing dragging on several hours — and well into happy hour time on the East Coast — it's possible some people may actually want to heed that advice.
Is Facebook a monopoly?
In a clever line of questioning, Sen. Lindsey Graham asked Zuckerberg to name his direct competitors.
When Zuckerberg began naming different categories of competitors who may overlap with what Facebook is doing, Graham stopped him to ask if there was any direct, comparable alternative to Facebook. He then flat-out asked Zuckerberg whether Facebook was a monopoly.
"It certainly doesn't feel like that to me," Zuckerberg said.
Graham followed up, asking if European regulations are right.
"I think they get things right," Zuckerberg said, prompting some audience laughter to break the tension.
Zuckerberg agrees Facebook is 'responsible for the content'
In his strongest statement yet on the topic, Zuckerberg acknowledged that Facebook is responsible for the content on its platform.
Facebook and other platforms have for years used "safe harbor" as a reason to absolve themselves of responsibility for what is on their platforms.
This admission could potentially spell changes for Facebook and other major tech companies if safe harbor is no longer accepted as the norm.
Robert Mueller's team has interviewed Facebook employees
Zuckerberg said special counsel Robert Mueller's team has interviewed some Facebook employees, but clarified that he hasn't been interviewed by the team.
"I want to be careful here, because that work is confidential. We are in open session and I don't want to reveal anything that is confidential," Zuckerberg said.
When asked if Facebook had been issued a subpoena, he said yes before backtracking.
"I have to clarify that I'm not sure that there are subpoenas, there may be," he said.
What hotel is Mark Zuckerberg staying at in Washington D.C.?
Landing the cable news sound bite of the hearing, Sen. Dick Durbin asked Zuckerberg if he would share the name of the hotel he slept at last night.
Zuckerberg paused, looking uncomfortable and then said he wouldn't. What about the people he direct messaged over the past week? Zuckerberg also declined to share that information.
Durbin's point was clear. These questions are why lawmakers and the public are concerned about how their data is shared on Facebook.
While Zuckerberg acknowledged that not every reads the privacy policies, he noted that users have choices in their privacy settings.
"I think everyone should have control over how their information is used," he said. "That is laid out in some of the documents, but, more importantly, you want to give people control in the product itself."
Facebook stock surges
The markets closed while Zuckerberg was still taking questions from 44 senators, but Facebook had a healthy trading day, with its stock closing at $165.04 — up 4.5 percent.
It was a much-needed boost for the company, which saw its share prices lose as much as 20 percent following the outbreak of the data harvesting scandal. In fact, it was Facebook's best day in two years on Wall Street, providing a much-needed dose of good news for the company.