Sandberg dismissed a suggestion reported by NBC News that Definers Public Affairs acted as a "fake news" operation to help Facebook deal with a public relations crisis over its handling of Russian misinformation.

"We absolutely did not pay anyone to create fake news. That, they have assured me, was not happening," Sandberg told CBS This Morning.

"And again, we're doing a thorough look into what happened but they have assured me that we were not paying anyone to either write or promote anything that was false. And that's very important."

She said she was unaware Facebook was working with Definers.

Facebook CEO Mark Zuckerberg said Thursday he too was in the dark about the firm, which according to a New York Times investigation sought to link Facebook critics to liberal financier George Soros.

The Hungarian-born US financier and philanthropist is a favorite target of nationalists and anti-Semitic conspiracy theorists.

Sandberg said Facebook was no longer working with Definers but added that "at the time, they were trying to show that some of the activity against us that appeared to be grassroots also had major organizations behind them."

"I did not know we hired them or about the work they were doing, but I should have," she in a post on her Facebook page.

"I have great respect for George Soros -- and the anti-Semitic conspiracy theories against him are abhorrent."

- 'Black ops' methods -

Soros is a founder of the Open Society Institute, whose president Patrick Gaspard stepped up criticism of Facebook in an interview Thursday.

"There are broad questions about the role that Facebook is playing in the integrity of our elections, our democracy," Gaspard told National Public Radio.

"And frankly now that we know that they ran this black ops operation, there are questions about why they're following the model of people like Vladimir Putin. This is profoundly dangerous and disturbing."

NBC News reported that a former employee of Definers, who asked not to be identified, described a website operated by the consulting group as an "in-house fake news shop" that sought to push stories into mainstream media.

A letter to Zuckerberg signed by four Democratic senators expressed concern over the latest revelations and raised the prospect of tough regulation of social media.

"We are gravely concerned by recent reports indicating that your company used contractors to retaliate against or spread intentionally inflammatory information about your critics," said the letter from Senators Amy Klobuchar, Mark Warner, Chris Coons and Richard Blumenthal.

"In addition, the staggering amount of data that Facebook has collected on both its users and people who have not subscribed to or consented to use of the platform, raises concern that the company could improperly or illegally use its vast financial and data resources against government officials and critics seeking to protect the public and our democracy."

Definers said in a statement Friday it did "basic media monitoring and public relations around public policy issues" for Facebook, as it does for many clients, and that it "was not hired by Facebook as an opposition research firm."

"A fraction of our work with Facebook included providing research and background information about critics -- both on the left and the right," the consulting group said on its website.

"This practice, standard across many industries, is based on researching public records and databases available to anyone."

bur-rl/ft

Facebook