More than 70 rights groups have asked Facebook to clarify its policies for removing content, especially at the behest of governments, alleging the company has repeatedly censored postings that document human rights violations.
In a letter sent to Facebook chief executive Mark Zuckerberg, the organisations criticise the social media company for cases in recent months where it has deleted content involving police violence, removed iconic imagery from the Vietnam War and briefly suspended accounts belonging to two Palestinian journalists.
"News is not just getting shared on Facebook: it's getting broken there," read the letter, whose signatories include the American Civil Liberties Union, Sierra Club, Center for Media Justice and SumOfUs.
"When the most vulnerable members of society turn to your platform to document and share experiences of injustice, Facebook is morally obligated to protect that speech," it continued.
Facebook's content policies have come under growing international scrutiny amid several controversial takedowns and reversals in recent months, including the company's handling of an iconic Vietnam war photo showing a naked girl burned by napalm.
A Facebook spokeswoman said the company was reviewing the letter.
"We welcome feedback from our community as we begin allowing more items that people find newsworthy, significant, or important to the public interest," the spokeswoman said.
Facebook announced last month it would begin weighing news value more heavily when deciding whether to block content.