As Facebook CEO, you don't know some key facts: US lawmaker to Mark Zuckerberg

In over 10 hours of questioning over the last two days, Zuckerberg was able to deflect most questions. Forty times the Facebook chief told lawmakers he had no answers at hand and would get back to them later. About one in three lawmakers got that response over the two days.

Democratic Representative Debbie Dingell expressed frustration with Zuckerberg's frequent promises to get back to lawmakers later in writing. "Some things are striking during this conversation," she said. "As CEO, you didn't know some key facts."

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During the hearing, the Facebook CEO conceded that regulation was the need of the hour.

"It is inevitable that there will need to be some regulation" of internet firms, Zuckerberg said, but he avoided any specifics.

During the hearing, the Facebook chief said he was among the nearly 87 million people whose personal information was improperly shared with political consultancy firm Cambridge Analytica. He gave no further details.

"How can consumers have control over their data when Facebook does not have control over the data?" asked Representative Frank Pallone of New Jersey at the beginning of Wednesday's hearing.

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

"I do imagine that we will find some apps that were either doing something suspicious or misusing people's data," he said.