Mark Zuckerberg made his highly anticipated debut before the Senate today during a marathon five-hour hearing before a joint session of the Commerce and Judiciary committees. Zuckerberg remained calm and level-headed throughout, and senators were mostly polite and deferential as they sought to understand how Facebook had inadvertently allowed the profiles of up to 87 million people to be collected by the political data-mining firm Cambridge Analytica.

In the weeks leading up to the hearing, Facebook made a series of announcements designed to demonstrate that it took the data leak seriously and was working to prevent it from happening again. Zuckerberg referred repeatedly today to these changes, which include making privacy shortcuts easier to find, restricting the data shared with developers when you log in using your Facebook account, labeling political ads and making them available for public inspection, and launching a bounty program to reward people who find examples of data misuse.

Facebook also sent Zuckerberg and his chief operating officer, Sheryl Sandberg, on a media tour to answer questions and hone their talking points. By the time today’s hearing began, Facebook had done what it could to ensure the day would feel light on news. Meanwhile, many senators still struggle to understand basic questions about how Facebook collects data and makes money. (Hint: not by selling that data to advertisers.)

Here are the five most notable developments from today’s hearings.

Zuckerberg had to confront Facebook’s monopoly power. When Sen. Lindsey Graham (R-SC) asked Zuckerberg to name his biggest competitor, Zuckerberg couldn’t name one. He was pressed repeatedly on Facebook’s large size, and at one point, he was asked whether Facebook was too powerful. Zuckerberg demurred. “It certainly doesn’t feel like that to me,” he said in response to Graham’s monopoly question. Senators do seem to be grappling with Facebook’s massive power in a way they haven’t before. But it’s not clear they have any coherent strategy to increase the amount of competition in the social media marketplace.

Zuckerberg won’t rule out a paid version of Facebook. The CEO took numerous questions about the company’s business model and whether it could truly protect users’ privacy given that it relies so heavily on collecting data about their lives and behavior. Multiple senators asked Zuckerberg whether he might consider a paid, ad-free version of Facebook in the future. He told Sen. Orrin Hatch (R-UT) that there would always be a free version of Facebook, suggesting a paid option might be possible. Later, he told another senator that a paid version would be worth thinking about.

Zuckerberg is leaning heavily on the future promise of artificial intelligence. Whenever asked about how Facebook would improve its moderation tools, Zuckerberg invoked the promise of AI to help Facebook quickly sort through hate speech and other problematic posts. It certainly seems possible that AI will improve Facebook’s content moderation efforts, but it remains unproven.

The conspiracy about Facebook targeting ads at you by snooping with your phone’s microphone is now part of the congressional record. For years now, Facebook has struggled to contain an urban legend that the company’s ad targeting is so effective because the company listens to your conversations in real time through your phone’s microphone. Thanks to Sen. Gary Peters (D-MI), this is now a matter of public record. “Yes or no, does Facebook use audio obtained from mobile devices to enrich personal information about users?” he asked. “No,” Zuckerberg said.

Senators don’t understand how Facebook works. Senators peppered Facebook with questions about the basic features of its data-collection and advertising practices. How does Facebook acquire data? How long does it keep that data? How can users control what data they share? These are important questions, and senators were surely speaking for the majority of Americans when they asked them. At the same time, they frittered away hours of testimony by asking the CEO questions that can be answered by Googling. And they mostly failed to answer deeper questions about how Facebook uses the data it collects. Of course, some senators argued that complexity itself is the problem. As Sen. John Kennedy (R-LA) put it — bluntly, if not helpfully — “your user agreement sucks.”

For more highlights of today’s hearing, read our live blog.