In the open letter, the advocacy groups, including the American Civil Liberties Union (ACLU), on Wednesday asked the social network to be more transparent about its censorship decisions and sought a report that compiles data on the company's censorship decisions, tech website CNET reported on Thursday.
In the letter, Facebook was accused of a lack of transparency and racially-biased censorship under which, it disproportionately censored posts from "people of colour" -- especially posts with political speech and critiques of law enforcement.
"Even as activists have been censored for political speech and for posting images critical of government actors -- including police officers -- Facebook's third-party complaint process has failed to prevent the spread of violent threats and harassment by white supremacist hate groups on your platform," the letter read.
The report pointed out that the "third-party complaint process" is Facebook's practice of hiring outside groups to sort through posts flagged as potential hate speech.
Last year, a woman used the Facebook's Live Video service to live-stream a video of her fiance, Philando Castile, after he was shot by police. Facebook temporarily removed the video and faced flak for doing so.
Similarly in September, Facebook initially took down a post that featured the iconic Vietnam War photo "Napalm Girl" because it featured a naked child.
Facebook, which has 1.79 billion users. later clarified that it only removed content if it celebrated or glorified violence, not if it's only graphic or disturbing.
"A Facebook spokeswoman said the company received the letter and is reviewing it," the report noted.