SAN FRANCISCO — Facebook and Twitter are stepping up their efforts to combat fake news.

Facebook said Thursday that it would end an experiment in six countries that separated posts from news sites and publishers from other material on the social network.

News organizations in the countries — Slovakia, Sri Lanka, Cambodia, Bolivia, Guatemala and Serbia — had said they were blindsided by the Facebook experiment when it began in October and complained that it had led to a surprising rise in misinformation.

Facebook stopped short of acknowledging that the changes made in the experiment, called Explore, contributed to that increase in misinformation, but the company said it could have done a better job communicating with publishers about what was happening.

“In surveys, people told us they were less satisfied with the posts they were seeing, and having two separate feeds didn’t actually help them connect more with friends and family,” Adam Mosseri, who heads Facebook’s News Feed, said in a statement posted online. “We also received feedback that we made it harder for people in the test countries to access important information, and that we didn’t communicate the test clearly.”

Facebook and other social media companies are grappling with their role in spreading false information propagated by Russians before the 2016 presidential election. And they face increasing pressure from regulators around the world to get a better handle on what happens on their networks, from misinformation to cyberbullying.

Measuring Twitter’s ‘health’

Also on Thursday, Jack Dorsey, chief executive of Twitter, said his company was looking for ways to measure the “health” of conversations on Twitter.

In a series of tweets, Dorsey acknowledged that Twitter has been home to a number of people who use the platform to harass, bully and mislead.

“We have witnessed abuse, harassment, troll armies, manipulation through bots and human-coordination, misinformation campaigns, and increasingly divisive echo chambers,” Dorsey wrote. “We aren’t proud of how people have taken advantage of our service, or our inability to address it fast enough.”

After Facebook released Explore in the six countries, it was quickly criticized by independent news sites and nongovernmental organizations. The changes, they said, made it harder for trusted sources of information to reach local populations. They also said the experiment had led to an increase in false and salacious news being shared in those countries.

Stuart White, editor of the The Phnom Penh Post, an English-language newspaper in Cambodia, said traffic from Facebook to his site had plummeted because of the changes to Facebook’s main News Feed — the scrolling stream of content that greets people when they log into the social network.

The experiment took place during a crackdown by the country’s leader, Hun Sen, on political opponents, activists and media, effectively transforming the struggling democracy into a one-party state. Journalists were arrested and newspapers were shut down. Facebook had emerged as an important, more independent channel for information.

“Whether or not Facebook likes it, it has established itself as the main market for news in places like Cambodia,” White said. “It would be helpful if they acknowledged that responsibility.” Fabiola Chambi, web editor of Los Tiempos, the main newspaper of the Bolivian city of Cochabamba, said Explore had reduced traffic to the site by 30 to 60 percent. Chambi said the Explore function was a constant source of confusion for publishers and readers who thought their access to news had been the subject of an experiment by a foreign company.

“It wasn’t an idea that was ever well-received, and garnered plenty of rejection,” she said. “And I would guess this had something to do with their decision to reverse course.”

However, the move did force the newspaper for the first time to think about life after Facebook. “We had to confront the declines and in the end found some creative ways to improve traffic and maintain a loyal audience,” Chambi said.

Facebook’s role as a kingmaker among publishers has played a central role in a debate over the responsibilities that social media companies have to police content on their own platforms.

‘Meaningful interaction’

Facebook has announced a series of changes to its News Feed as it looks to increase “meaningful interaction” on its site by drawing attention to content from family and friends while de-emphasizing content from brands and publishers. Those changes will be introduced gradually around the world.

Facebook has said those News Feed changes were not related to Explore, but there are similarities in the way the company is looking to downplay material from publishers.

While Facebook has said it was emphasizing meaningful interactions, Dorsey said Twitter was determined to find “holistic and fair solutions” to its problems.

The company recently rewrote its user rules and guidelines, and has begun punishing well-known Twitter users who violate those rules by removing their “account verification” status or barring them altogether.

It was unclear how either Twitter or Facebook will measure the new method by which they said they will judge their platforms, or how they will affect the billions of people who use them.