A nonprofit founded by billionaire philanthropist George Soros has posted a scathing letter on Facebook about the social media company's reported use of "a Republican opposition research firm to stir up animus toward" Soros.

The open letter to Sheryl Sandberg, Facebook's chief operating officer, was posted hours after The New York Times published a lengthy investigation into Facebook's reluctance to deal with revelations that Russian operatives leveraged the social media site as part of a complex 2016 election influencing campaign.

The Times reports in the article that after news of the meddling broke, Facebook employed a Republican opposition-research firm, Definers Public Affairs, "to discredit activist protesters, in part by linking them to the liberal financier George Soros," while "lobbying a Jewish civil rights group to cast some criticism of the company as anti-Semitic."

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In the letter to Sandberg, Patrick Gaspard, president of Soros' nonprofit Open Society Foundations, accuses Facebook of promoting "distortions" about Soros.

"As you know, there is a concerted right-wing effort the world over to demonize Mr. Soros and his foundations, which I lead—an effort which has contributed to death threats and the delivery of a pipe bomb to Mr. Soros' home. You are no doubt also aware that much of this hateful and blatantly false and anti-Semitic information is spread via Facebook," Gaspard wrote in the letter, under the headline, "Facebook's Smear Campaign Against George Soros."

"The notion that your company, at your direction, actively engaged in the same behavior to try to discredit people exercising their First Amendment rights to protest Facebook's role in disseminating vile propaganda is frankly astonishing to me," he wrote.

Facebook did not respond to an email from CBS News seeking a response directly related to Gaspard's letter. In a statement released by Facebook that did not mention Soros, the company pushed back against The New York Times' reporting on its relationship with Definers.

"The New York Times is wrong to suggest that we ever asked Definers to pay for or write articles on Facebook's behalf – or to spread misinformation," Facebook said, noting that Definers had openly done media communications work for the company. "Definers did encourage members of the press to look into the funding of 'Freedom from Facebook,' an anti-Facebook organization. The intention was to demonstrate that it was not simply a spontaneous grassroots campaign, as it claimed, but supported by a well-known critic of our company."

Soros, who is often the target of anti-Semitism, has been critical of Facebook before. In a Jan. 25 speech at the World Economic Forum in Davos, Switzerland, Soros called Facebook and Google a "menace" and called for "regulatory authorities to protect society against them."

Facebook said in its statement that it ended its contract with Definers Wednesday night. In a statement to CBS News, a Definers spokesperson said the company "proud to have partnered with Facebook over the past year on a range of public affairs services."

"All of our work is based on publicly-available documents and information. The document referenced in the Times story regarding the anti-Facebook organization's potential funding sources was entirely factual and based on public records, including public statements by one of its organizers about receiving funding from Mr. Soros' foundation," the Definers spokesperson said.

As news of Facebook's use of Definers reached Capitol Hill, one senator implied that Facebook's efforts to push back against critics may add to the company's woes.

Sen. Amy Klobuchar, D-Minnesota, told reporters she will be sending a letter to Facebook and the Justice Department.

"I think it's very important that we get to the bottom of that, because it's very clear they went all out against their critics," Klobuchar said.