WASHINGTON • Facebook's No. 2 executive Sheryl Sandberg, long seen as the "adult" at the youthfully-managed firm, has found herself the centre of controversy over her role in pushing back at a growing chorus of criticism of the social media giant.

A prominent feminist and author with strong political connections, Ms Sandberg has drawn fire in particular over an embarrassing effort to probe billionaire investor George Soros, after he assailed the online network as a "menace to society".

Facebook last Thursday acknowledged that Ms Sandberg had asked her staff to conduct research on the Hungarian-born billionaire, following his remarks early this year, out of concern that he held a "short" position that would let him profit from a decline in the firm's share price.

"Mr Soros is a prominent investor and we looked into his investments and trading activity related to Facebook," a spokesman said.

"That research was already underway when Sheryl sent an e-mail asking if Mr Soros had shorted Facebook's stock."

Ms Sandberg previously denied the use of nefarious tactics against Facebook critics. The company has come under fire for enabling the spread of misinformation, including Russian-led propaganda, during the 2016 US election campaign.

The 49-year-old Ms Sandberg has long been seen as a stabilising force at Facebook, led by 34-year-old chairman and chief executive Mark Zuckerberg, whose early mantra for the firm had been to "move fast and break things".

Among the tech whizz kids, Ms Sandberg as chief operating officer was seen as offering a steadier hand as a result of her background working for former US Treasury secretary Larry Summers and the philanthropic arm of Google.

Author of the feminist bestseller Lean In, Ms Sandberg also drew attention in 2015 after the sudden death of her husband, management consultant David Goldberg, at the age of 47.

Earlier this year she was dispatched to testify in Congress to defend Facebook's efforts in dealing with misinformation and manipulation, in the wake of a scandal over user data hijacked by the Cambridge Analytica consultancy linked to Mr Donald Trump.

But the latest controversy over opposition research raised questions about whether Ms Sandberg is living up to her role.

Mr Roger Kay, an analyst and consultant at Endpoint Technologies Associates who follows the tech sector, said the moves by Facebook and Ms Sandberg showed "extraordinarily poor judgment" from a corporate governance view.

"She's not behaving as the adult she should have been," Mr Kay said. "Maybe they need another adult in the room."

New York City Comptroller Scott Stringer, whose office has shares in Facebook, said the latest developments highlight the need for an independent board chair to oversee Mr Zuckerberg and Ms Sandberg.

"Where is Facebook's board? The silence from the board of directors in the midst of continuous scandals is deafening," Mr Stringer tweeted following the report on research efforts on Mr Soros.

Others defended Ms Sandberg, saying she was correct to look into powerful interests opposing Facebook and noting that Mr Soros - who notoriously made a fortune by betting against the British pound in 1992 - is known as a highly sophisticated investor.

Facebook "management would have been absolutely remiss to not look into whether Soros was (again) manipulating a security for personal profit, to the detriment of the FB shareholder", tweeted Mr Antonio Garcia Martinez, a former Facebook product manager and author of a book on Silicon Valley.

"He's a hedge fund pirate answerable to no one, as bad as anyone on Wall Street. Why wouldn't they check?"

Some questioned whether Ms Sandberg's story had shifted since she said earlier last month that she was unaware that Facebook had hired Definers Public Affairs to conduct research on company critics.

"Despite public claims to the contrary, Sheryl Sandberg knew @facebook had hired Definers to run opposition research," tweeted the group Freedom From Facebook, one of the activist organisations leading efforts to regulate or break up the social network.

AGENCE FRANCE-PRESSE