One of George Soros's top officials has called on US politicians to probe Facebook, after the social network confirmed that it had hired a PR firm to make claims about the financier.
The head of Mr Soros's grant-making network claimed Facebook had smeared the philanthropist, adding "this needs independent, congressional oversight".
Sheryl Sandberg, Facebook's deputy chief, has also clarified her role.
She said she had been told of the PR firm but had not remembered its name.
The latest developments follow the social network's decision to publish a memo by its departing communications chief, Elliot Schrage.
In it, he confirmed Facebook had directed the PR firm Definers to investigate Soros's links to the Freedom from Facebook campaign, which is seeking the company's break-up.
Mr Schrage added that related documents were then sent to journalists on Facebook's behalf.
The memo had originally been sent to Facebook's staff and had already been leaked to the news site Techcrunch .
But its re-publication by Facebook represented the first confirmation that Definers had not been engaged in a rogue operation.
Patrick Gaspard, president of Mr Soros's Open Society Foundations, responded by calling for an official investigation, and suggested that Facebook had deliberately timed the revelation to coincide with the US Thanksgiving holiday.
So @facebook decides to drop a turkey on Thanksgiving eve, with admission that Definers was tasked by company leadership to target and smear George Soros because he publicly criticized their out of control business model. Sorry, but this needs independent, congressional oversight— Patrick Gaspard (@patrickgaspard) November 21, 2018
In addition to publishing Mr Schrage's message, Facebook also issued an update from its chief operating officer Sheryl Sandberg.
A week ago, she wrote that she had not known that Facebook had hired Definers, the PR firm involved, nor knew about the work it had done on her company's behalf.
Chief executive Mark Zuckerberg had also described both himself and his deputy as having been kept "out of the loop".
Ms Sandberg now acknowledges that she had in fact been told about the company.
"Last week, I didn't remember a firm called Definers," she wrote.
"I asked our team to look into the work Definers did for us and to double-check whether anything had crossed my desk,
"Some of their work was incorporated into materials presented to me and I received a small number of emails where Definers was referenced."
Ms Sandberg added that she rejected claims that her firm had sought to put the spotlight on Mr Soros in order to exploit racist conspiracy theories against him.
"It was never anyone's intention to play into an anti-Semitic narrative against Mr Soros or anyone else. Being Jewish is a core part of who I am and our company stands firmly against hate," she wrote.
'Menace to society'
Some company-watchers have suggested that Facebook's decision to publish the memo marks an attempt to protect Ms Sandberg.
She had reportedly angered "many people" within her firm by attempting to distance herself from the controversy, according to an earlier report by the Wall Street Journal, which said she had a reputation for closely managing Facebook's media strategies.
This looks like Facebook scapegoating Elliot Schrage when he's on the way out anyway. https://t.co/4abRhzt65J— Olivia Solon (@oliviasolon) November 22, 2018
Schrage makes a convenient answer to "Who'll get fired for the Definers fiasco" since he's already announced he's resigning https://t.co/vObyVsCU5c— Josh Constine (@JoshConstine) November 21, 2018
Given Facebook’s attempts to pin all this on the guy who’s leaving anyway and get Kaplan/Sandberg/Zuck off the hook, I’d love to see Congress explore what kind of parting gift Elliot Schrage eventually gets. https://t.co/DRslsb8SMU— Jamison Foser (@jamisonfoser) November 21, 2018
Mr Schrage wrote that he took responsibility for the affair.
He said that his team had only asked Definers to look into Mr Soros after the billionaire had described the social network as being a "menace to society".
But Mr Schrage provided no evidence that Mr Soros was more directly involved in the campaign.
And although he acknowledged that he "should have known of the decision to expand [Definers'] mandate," he did not address specifically how he thought the PR firm had overstepped the mark.
His memo did, however, touch on the fact that Facebook has become prone to leaks.
"I'm deeply disappointed that so much internal discussion and finger pointing has become public," Mr Schrage wrote.
"This is a serious threat to our culture and ability to work together in difficult times."
The New York Times has also published a follow-up report to its original expose about Facebook and Definers.
It contains claims that Definers also engaged in a campaign against Apple at a time the PR firm was working for the chip-maker Qualcomm - the two tech firms are involved in a long-running legal battle.
The report alleges that Definers promoted the idea that Tim Cook might seek to become US President in 2020, which the newspaper suggested had been done to undermine the Apple chief executive's relationship with President Trump.
The BBC has contacted all three companies for comment but has not had a response.
The NYT did, however, publish a statement from Definers saying its work was "absolutely no different than what public affairs firms do every day for their clients across industries and issues across the country".