The 33-year-old Zuckerberg frequently argued, for example, that Facebook's 2 billion users - not the company - own the data they share via its network, and can decide whenever they wish to prevent Facebook from having it. That line almost worked, until Democrats Kathy Castor and Ben Lujan pointed out that the company is collecting personal data on people who don't use the social network and have never signed a privacy agreement.

Zuckerberg said that the company tracks non-users for "security purposes", without elaborating. But there were further questions about the information Facebook collects on members who are logged out of the social network, on websites that track Facebook users in hidden ways - an option the company openly touts to advertisers but most users aren't aware of. Zuckerberg didn't know how to answer some of the queries about that tracking, and told the committee that his staff would follow up.

Members of Congress in both houses prodded Zuckerberg frequently about failure to police the content on his platform, too. In some instances, they worried that Facebook wasn't taking down harmful content fast enough, like fake profiles of people who are stealing pictures from real people, as well as posts seeking to recruit for terrorist ideologies.

"Where's your accountability?" David McKinley, a Republican, asked Zuckerberg after demonstrating that there were still posts on Facebook illegally offering opiate drugs.

Several Republicans also strongly voiced their concerns on the opposite end of the spectrum - that over-regulating or policing of the platform would lead to anti-conservative bias and a general silencing of offensive opinions.

Responding to politicians' complaints about how Facebook handles content, Zuckerberg explained that the company has the opportunity to better moderate its site once it develops an artificial intelligence solution, which may take months or years. Such a system might have the potential to block bad posts before they spread, without humans needing to make subjective decisions in individual cases. But Zuckerberg didn't mention that artificial intelligence is just a term for a computer program designed by humans who may have their own biases, just like Facebook's algorithm.

Senator Lindsey Graham, a Republican from South Carolina, questioned Zuckerberg about Facebook's power more broadly: is the social network a monopoly? Zuckerberg came prepared with a data point about how most people use eight different apps to keep in touch with friends and family - a number that may include email and text messaging, which aren't direct competitors of Facebook. He also didn't say how many of those eight might be Facebook's popular subsidiary apps, which include Instagram, Messenger and WhatsApp.

"Contrary to Mr Zuckerberg's assertion, Facebook is a virtual monopoly and monopolies need to be regulated," Graham said after the hearing.

Luckily for California-based Facebook, US lawmakers aren't aligned on exactly what regulation would look like, or if written laws or rules are even necessary. Some said they wanted Facebook to try its hand at self-regulation first, while others asked Zuckerberg to propose policies. Zuckerberg said he was open to the idea of regulation, but would need to see the specifics of any bill in order to agree wholeheartedly. Congress is unlikely to get it done during this session.

For now, legislators will be asking many follow-up questions - first of all about the problem that brought Zuckerberg to Congress. Information from as many as 87 million users was obtained by an app developer claiming to be doing academic research, who then sold those personal details to Cambridge Analytica, a British firm with ties to US President Donald Trump's 2016 campaign. Zuckerberg said the company is auditing tens of thousands of apps to find other such leaks, and will inform affected users when it identifies them.

"After more than a decade of promises to do better, how is today's apology different?" Senator John Thune, the Republican chairman of Senate Commerce, asked Zuckerberg. "And why should we trust Facebook to make the necessary changes to ensure user privacy and give people a clearer picture of your privacy policies?"

It's not just about privacy, said John Sarbanes, a Democrat. "A lot of Americans are waking up to the fact that Facebook is becoming sort of a self-regulated superstructure for political discourse," he said.

The Washington Post