Facebook CEO Mark Zuckerberg fielded 10 hours of questions over two days from nearly 100 U.S. lawmakers and emerged richer, as the stock showed its stance positively and lawmakers remain puzzled on a possible regulation related to social media.

Questions to Zuckerberg ranged from data privacy to prescription drug sales to employee diversity, althoug they were mostly focused on the challenge of trying to grasp and confront Facebook's power in consumers' lives.

Facebook's investors were cheered by Zuckerberg's calm responses to barrages of questions. The company's shares closed up 0.78 percent on Wednesday after rising 4.5 percent Tuesday. Over the two days, the value of Zuckerberg's stake in the company grew about $3 billion.

During the session, 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.

He 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 representatives 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.

Responding to lawmakers' 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. 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.

Lawmakers aren't aligned on exactly what social media 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.

Zuckerberg undercut his consistent message that Facebook users have control of their data. He said he was among the nearly 87 million people whose personal information was improperly shared with political consultancy Cambridge Analytica.

He said it would take "many months" to complete an audit of other apps that might also have improperly gathered or shared users' data.

Instagram to Allow Users Download Their Data

In related news, Facebook's Instagram will soon allow its users to download a copy of all the content they have uploaded on the photo-sharing platform.

The disclosure comes amid global concerns about the privacy of users' information on social media platforms and the amount of user data that companies keep.

While Facebook has allowed its users to download their photos, messages, clicked advertisements and a log of all their activity on the social networking platform since at least 2010, Instagram has lacked any such feature.

The tool will probably help Instagram comply with upcoming European privacy laws that require data portability.

EU's Top Court may Probe Facebook

Seperately, the Irish High Court on Thursday asked the EU's top court for a detailed assessment of whether firms such as Facebook can easily transfer Europeans' data to the United States.

The inquiry stems from a case brought by an Austrian privacy activist against the methods used by Facebook to store user data on U.S. servers following revelations in 2013 of mass U.S. surveillance practices.

The High Court's referral asks the Court of Justice of the EU (ECJ) if the Privacy Shield - under which companies certify they comply with EU privacy law when transferring data to the United States - does in fact mean that the United States "ensures an adequate level of protection".

EU data protection law prohibits personal data being transferred to a country with inadequate privacy protections.

Facebook has until April 30 to lodge an application to block the referral.

The Irish court also asked the ECJ whether European data protection authorities ought to suspend data flows if the company moving the data outside the EU is subject to "surveillance laws" that conflict with the EU's right to privacy.