The new revelation, initially reported by the New York Times then confirmed by the company to others, undermines Sandberg's attempts to distance herself from Facebook's hiring of a conservative research outfit to find discrediting information on the liberal philanthropist.
Sandberg, who was at the forum - but was not present for Soros's speech, according to a person who attended it - asked for an examination into why Soros had criticised the tech companies and whether he stood to gain financially from the attacks.
The PR firm was hired to look into links between Soros and purportedly grassroots groups, such as "Freedom from Facebook", that publicly lambasted the social network owner. "The silence from the board of directors in the midst of continuous scandals is deafening", Stringer tweeted following the Times report on research efforts on Soros.
"That research was already underway when Sheryl sent an email asking if Mr Soros had shorted Facebook's stock. But, as she said before, she takes full responsibility for any activity that happened on her watch".
Others defended Sandberg, saying she was correct to look into powerful interests opposing Facebook and noting that Soros - who notoriously made a fortune by betting against the British pound in 1992 - is known as a highly sophisticated investor.
Author of the feminist bestseller "Lean In", Sandberg also drew attention in 2015 after the sudden death of her husband David Goldberg at age 47.
Schrage joined Zuckerberg and Sandberg in stressing that Definers was not hired to create or spread false stories to help Facebook.
Schrage taking the hit for the controversy was seen by some as convenient, since he had said he was leaving the social network after working there for more than a decade.
"We had not heard such criticism from him before and wanted to determine if he had any financial motivation", Shrage said discussing Soros. Facebook wanted to deflect some of the mounting criticism it faced after both providing the vehicle for Russian agents to disrupt the USA 2016 presidential election and leaking the personal data of millions of users to the Trump campaign consultancy Cambridge Analytica.
Facebook hired Definers past year to monitor media coverage and then expanded its role to include campaign-style research and other public relations work.
That activity included having the PR firm label several activist groups, Freedom From Facebook and Color of Change, as pawns of Soros.