Revelations about the US government's secret surveillance programs has had a big impact on "trust metrics" of Internet companies like Facebook, the social network chief Mark Zuckerberg said Wednesday.

Speaking at a Washington forum, Zuckerberg said Facebook would keep pressing for more transparency about the National Security Agency programs disclosed in recent months believed to collect vast amounts of data on Internet users.

Zuckerberg said Facebook keeps a close eye on "trust metrics," or how users feel about the company over time, and that the revelations about the NSA PRISM program had a major impact on trust.

"There's a lot of times we will put a statement out (on issues), and that stuff tends not to move the needle on trust. The NSA stuff did," he told the forum sponsored by The Atlantic magazine.

"The trust metrics... went down when PRISM came out, so this is a really big deal."

Facebook and other Internet firms have filed court petitions seeking to disclose more information about the role of the companies in the NSA program, in the hopes this would ease concerns among Internet users.

"The more transparency there is, the better everyone would feel about it," he said.

"From reading all the press you couldn't get a sense whether the number of the requests the government makes is closer to a thousand or closer to 100 million."

He said Facebook's "transparency report" indicated it received "on the order of 9,000" requests for data from officials in the United States, but the company was not allowed to disclose how many of these were national security requests.

Zuckerberg declined to comment on the value of the surveillance programs in protecting the nation.

"I don't know all the things they are doing to protect our safety," he said.

"My general belief is that the more transparent they are about what they are doing, the more comfortable the public will be about it."

The comments were less harsh than a week earlier, when Zuckerberg appeared at a California conference.

"It is our government's job to protect all of us and also protect our freedoms and protect the economy and protect companies," he said at the San Francisco event. "Frankly, I think the government blew it."

Zuckerberg said he was in Washington to press immigration reform through his advocacy group Fwd.us.

He said he did not want to separate the debate on immigration for "high skills" people sought in the tech sector from the broader question of undocumented immigrants, estimated at 11 million.

"It's a much bigger problem than the high skills piece," he said. "Eleven million people are being treated unfairly."