Facebook said on Friday it would tighten its ban on hateful content in ads and start labelling newsworthy content that violates the social media company's policies.

The policy changes come as an advertising boycott campaign by several US civil rights groups gains traction. On Friday, Unilever PLC said it would stop US ads on Facebook, Instagram and Twitter for the rest of the year, citing "divisiveness and hate speech during this polarised election period in the US".

Chief Executive Mark Zuckerberg said in a live-streamed company town hall that Facebook would ban ads that claim people from groups based on race, religion, sexual orientation or immigration status are a threat to physical safety, health or survival.

Unilever, which owns brands like Dove Soap and Lipton tea, joins the growing ad boycott against Facebook as part of the "Stop Hate for Profit" campaign started after the death of George Floyd. The effort urges Facebook, which owns Instagram, to do more to stop hate speech and misinformation.

Shares of Facebook and Twitter both fell more than 7 per cent.

"Continuing to advertise on these platforms at this time would not add value to people and society. We will be monitoring ongoing and will revisit our current position if necessary," Unilever said in a statement.

The campaign specifically asks businesses not to advertise on Facebook's platforms in July, though Twitter has also long been under pressure to clean up alleged abuses and misinformation on its platform.

"We have developed policies and platform capabilities designed to protect and serve the public conversation, and as always, are committed to amplifying voices from under-represented communities and marginalised groups," said Sarah Personette, vice-president for Twitter's Global Client Solutions.

"We are respectful of our partners' decisions and will continue to work and communicate closely with them during this time."

More than 90 advertisers including Verizon Communications Inc and The North Face, a unit of VF Corp, have joined the campaign, according to a list by ad activism group Sleeping Giants, a partner in the campaign.

Earlier this week, ice-cream maker Ben & Jerry's, a unit of Unilever, said it would pull its Facebook and Instagram ads in the United States.

In a statement, a Facebook spokeswoman pointed to its civil rights audit and investments in Artificial Intelligence that allows it to find and take action on hate speech.

"We know we have more work to do, and we'll continue to work with civil rights groups, GARM [the Global Alliance for Responsible Media], and other experts to develop even more tools, technology and policies to continue this fight," she said.