By Agence France-Presse
Advocacy and rights group took direct aim at Zuckerberg and Sandberg for Facebook enlisting Definers Public Affairs to conduct research on company critics.
"You retained the services of Definers Public Affairs to investigate, undermine and attack our allies, mimicking the tactics of the worst, disreputable political operatives and hate groups," read the letter.
"It's an absolute disgrace that Facebook sought to deflect criticism and discredit advocates by exploiting anti-Semitic campaigns against philanthropist George Soros."
The letter called on Zuckerberg and Sandberg to step down from the Facebook board as long as they serve as top executives, and to expand the board with members reflecting the social network's diverse global community.
The 32 groups signed to the letter included Arab American Institute; Bend the Arc Jewish Action; Freedom From Facebook, Media Matters for America; MomsRising; MoveOn NAACP; National LGBTQ Task Force, Southern Poverty Law Center, and The Sikh Coalition.
The letter was sent the same day that the NAACP kicked off a "#LogoutFacebook" boycott of the social network to protest treatment of non-whites at the social network and in the workplace.
The NAACP called on the Congress to further investigate Facebook in the wake of a Senate Intelligence Committee report revealing that the Russian influence campaign "made an extraordinary effort to target African-Americans."
"Facebook's engagement with partisan firms, its targeting of political opponents, the spread of misinformation and the utilization of Facebook for propaganda promoting disingenuous portrayals of the African American community is reprehensible," NAACP president Derrick Johnson said in a release.
Facebook's number two executive Sheryl Sandberg, long seen as the "adult" at the youthfully-managed firm, has found herself the center 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, Sandberg has drawn fire in particular over an embarrassing effort to probe Soros, the billionaire investor, after he assailed the online network as a "menace to society."
Chief executive Zuckerberg has also been targeted by critics who consider him accountable for the actions of the company.
Reports commissioned by the US Senate and unveiled Monday said that propaganda campaigns conducted by Russia across a gamut of social networks before the US presidential election in 2016 included tactics aimed at discouraging black people from voting.
Facebook released an internal civil rights audit on Tuesday, along with a message from Sandberg vowing that they are a priority at the social network.
"As a direct response to feedback from civil rights advocates, we are focusing on voter suppression as a distinct civil rights challenge and will continue this work to be better prepared for future elections," Sandberg said.