Eddie Vale, a spokesman for Freedom from Facebook, said he was skeptical of the company’s account.

“In light of Sandberg’s continuously changing story on the Soros research, there’s no way their denials about attacking other critics can be taken at face value,” Mr. Vale said. “Facebook must immediately release any emails and any research about targeting the Freedom from Facebook coalition or any member organizations.”

The revelation complicates Ms. Sandberg’s shifting explanations of her role in Facebook’s decisions to hire Definers and go on the offensive against the social network’s growing legion of critics. Ms. Sandberg at first denied knowing that Facebook had hired Definers, before acknowledging in a post last week that some of the company’s work for Facebook had crossed her desk.

Image Elliot J. Schrage, who oversaw Facebook’s communications team and is leaving the company, previously took responsibility for hiring the firm that looked into Mr. Soros. Credit... James Lawler Duggan/Reuters

In that post, Ms. Sandberg did not explicitly deny that she had asked for research into Mr. Soros. Instead, a deputy who oversaw the communications team but is now leaving the company, Elliot J. Schrage, took responsibility for hiring Definers and initiating Definers’ investigation into Mr. Soros. It is unclear what, if any, involvement Ms. Sandberg had in that ultimate response to Mr. Soros.

“We had not heard such criticism from him before and wanted to determine if he had any financial motivation,” Mr. Schrage said of Mr. Soros. “Definers researched this using public information.”

Facebook has defended its inquiries into Mr. Soros as a prudent and necessary step for any public company under attack by a high-profile figure — particularly one like Mr. Soros, a onetime currency trader who made a fortune in the 1990s betting against the British pound.

But the revelations are likely to escalate pressure on Ms. Sandberg, an embattled Silicon Valley star and feminist author.