Although Zuckerberg, 33, had never testified at a congressional hearing before, he succeeded in deflecting questions like a Washington veteran.
Forty times the internet mogul 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."
On one point, 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 gave no further details.
His admission that even the company's tech-savvy founder was unable to protect his own data underscored the problem Facebook has in persuading sceptical lawmakers that users can easily safeguard their own information and that further legislation governing Facebook is unnecessary.
The Cambridge Analytica issue was the reason Zuckerberg was on Capitol Hill, answering questions about how that company - which has counted US President Donald Trump's election campaign among its clients - got hold of data about Facebook users.
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.
Wearing a dark suit and tie and politely prefacing almost every remark with "Congressman" or "Congresswoman," Zuckerberg appeared even more controlled than he did yesterday when he testified before senators.
He refrained from cracking jokes and flashed few smiles.
The performance had favourable results. Facebook shares closed up 0.78 per cent after rising 4.5 per cent yesterday.
Over the two days, the value of Zuckerberg's stake in the company grew by about US$3 billion ($4b).
- Reuters, AAP
