By Nick Wingfield, Mike Isaac and Katie Benner, New York Times

During the past week, two of the world’s biggest internet companies have faced mounting criticism over how fake news on their sites may have influenced the presidential election’s outcome.

On Monday, those companies responded by making it clear that they would not tolerate such misinformation by taking pointed aim at fake news sites’ revenue sources.

Google kicked off the action Monday afternoon when the Silicon Valley search giant said it would ban websites that peddle fake news from using its online advertising service. Hours later, Facebook, the social network, updated the language in its ad policy, which already said it will not display ads in sites that show misleading or illegal content, to include fake news sites.

“We have updated the policy to explicitly clarify that this applies to fake news,” a Facebook spokesman said in a statement. “Our team will continue to closely vet all prospective publishers and monitor existing ones to ensure compliance.”

Taken together, the decisions were a clear signal that the tech behemoths could no longer ignore the growing outcry over their power in distributing information to the American electorate.

Facebook has been at the epicenter of that debate, accused by some commentators of swinging some voters in favor of President-elect Donald Trump through misleading and outright wrong stories that spread quickly via the social network. One such false story claimed that Pope Francis had endorsed Trump.

Google did not escape the glare, with critics saying the company gave too much prominence to false news stories. On Sunday, the site Mediaite reported that the top result on a Google search for “final election vote count 2016” was a link to a story on a website called 70News that wrongly stated that Trump, who won the Electoral College, was ahead of his Democratic challenger, Hillary Clinton, in the popular vote.

By Monday evening, the fake story had fallen to No. 2 in a search for those terms. Google says software algorithms that use hundreds of factors determine the ranking of news stories.

“The goal of search is to provide the most relevant and useful results for our users," Andrea Faville, a Google spokeswoman, said in a statement. "In this case, we clearly didn’t get it right, but we are continually working to improve our algorithms.”

Facebook’s decision to clarify its ad policy language is notable because Mark Zuckerberg, the social network’s chief executive, has repeatedly fobbed off criticism that the company had an effect on how people voted. In a post on his Facebook page over the weekend, he said that 99 percent of what people see on the site is authentic, and only a tiny amount is fake news and hoaxes.

“Overall, this makes it extremely unlikely hoaxes changed the outcome of this election in one direction or the other,” Zuckerberg wrote.

Yet within Facebook, employees and executives have been increasingly questioning their responsibilities and role in influencing the electorate, The New York Times reported over the weekend.

Facebook’s ad policy update will not stem the flow of fake news stories that spread through the news feeds that people see when they visit the social network.

Facebook has long spoken of how it helped influence and stoke democratic movements in places like the Middle East, and it tells its advertisers that it can help sway its users with ads. Facebook reaches 1.8 billion people around the globe, and the company is one of the largest distributors of news online. A Pew Research Center study said that nearly half of American adults rely on Facebook as a new source.

Google’s decision Monday relates to the Google AdSense system that independent web publishers use to display advertising on their sites, generating revenue when ads are seen or clicked on. The advertisers pay Google, and Google pays a portion of those proceeds to the publishers. More than 2 million publishers use Google’s advertising network.

For some time, Google has had policies in place prohibiting misleading advertisements from its system, including promotions for counterfeit goods and weight-loss scams. Google’s new policy, which it said would go into effect “imminently,” will extend its ban on misrepresentative content to the websites its advertisements run on.

“Moving forward, we will restrict ad serving on pages that misrepresent, misstate or conceal information about the publisher, the publisher’s content or the primary purpose of the web property,” Faville said.

Faville said the policy change had been in the works for a while and was not in reaction to the election.

It remains to be seen how effective Google’s new policy on fake news will be in practice. The policy will rely on a combination of automated and human reviews to help determine what is fake.

Although satire sites like The Onion are not the target of the policy, it is not clear whether some of them, which often run fake news stories written for humorous effect, will be inadvertently affected by Google’s change.

To Learn More:

Facebook at Center of Debate over Influence of Fake News on Election (by Barbara Ortutay, Associated Press)

Clinton to Win Popular Vote for U.S. President, While Trump’s “Rigged” Electoral College System Landed Him the Job (by Stephen Ohlemacher, Associated Press)

U.S. Presidential Election Further Exposes Great Divide between Online and Mainstream Media (by John Herrman, New York Times)

Rise in Social Media’s Power Reveals Growing Threat to Democracy: Fake News (by Jim Rutenberg, New York Times)

Truth Viewed as Victim, Not Beneficiary, of the Internet (by Farhad Manjoo, New York Times)