FREEDOM OF expression is a bedrock of American democracy, but its irresponsible exercise can distort and destabilize our politics. That reality is now at the center of attention in the social media world as Facebook and others confront questions about their role in spreading fake news and false information before and after the presidential campaign. The social media companies must not duck this issue. It goes to the heart of an open society, and also is the key to their own credibility and survival.

It wasn’t all that long ago that the Internet was a wild west, where information ran free. As Facebook, Google and Twitter grew, they celebrated this as a virtue, insisting they were technology companies built on algorithms and not news media outlets. But on the road to becoming true global giants, Facebook and the others took on enormous new power to shape the information that people consume. The latest Washington Post-Schar School national poll shows that when people are asked where they got their election news, 56 percent said television, and 30 percent said the Internet. Among the sources on the Internet, 32 percent of the respondents identified social media, 29 percent said news organizations’ websites, and 15 percent said Google or other search engines.

A distinction must be drawn between personal posts, which are best left largely unfettered, and the news feed posts that can quickly go viral, accelerated by algorithms that respond to user engagement. When these posts suddenly explode and reach millions of people, social media essentially become news media. There is serious abuse when the content is false, such as one post during the campaign saying the pope had endorsed Donald Trump (he didn’t), or a top Google search result the other day that pointed to a fake site reporting Trump had won the popular vote (he has not).

Fake news is dangerous mischief and takes advantage of the fact that social media generally rely on rapid-fire algorithms and not deliberate human editing. The social media services must adjust to the reality that they now are news media outlets to some extent; that means relying more on human editors to weed out the fake news. The task is delicate and requires balanced judgment. The aim must be not to censor free expression or favor certain political views, but to guard against deception and fraud. In recent days Facebook and Google announced they are cutting off advertising on sites carrying content that is illegal, misleading or deceptive; that’s a good first step.

Since the biggest social media outlets now span the globe, their operating standards must showcase the best of democracy at work, and avoid giving tyrants any impetus to crack down on dissent and free expression. This is a job for people, not algorithms.

Social media have good reason to act here in order to protect their own future. If users conclude that the information highlighted by Facebook, Google and Twitter is consistently ureliable, they will look elsewhere. But society at large also has a big stake. The Internet has become a vital forum for democratic debate; it is essential that that interchange not be warped by propaganda and lies.