One of George Soros’s lieutenants 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.

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 written 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.”