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We're starting today with an issue for millions of people, especially through social media. The wildfire spread of fake news. These are stories you read about that appeared to be factual but in fact have no basis in fact at all.

For example, before the U.S. presidential election, a number of false stories circulated online. What appeared to show Democratic presidential nominee Hillary Clinton about to receive thousands of fraudulent ballots in her favor. One appeared to show a quote from 1998 in which Republican presidential nominee Donald Trump trashed the very people who'd become his own voters.

To be clear, neither of these stories was true. But they still went viral. Experts say making sure a story is from a reliable news source, watching out for headlines that don't match article itself and avoiding sharing information from a single site you've never heard of can help curb a growing problem.

BRIAN STELTER, CNN SENIOR MEDIA CORRESPONDENT: Made up false stories are polluting people's Facebook timelines and Twitter streams.

And getting worse. Even President Obama is raising the alarm.

BARACK OBAMA, PRESIDENT OF THE UNITED STATES: If we are not serious about facts and what's true and what's not — then, we have problems.

STELTER: These problems are not brand new, but they're becoming a lot more prevalent.

PROFESSOR DAN GILLMOR, ARIZONA STATE UNIVERSITY: We have an epidemic of false information racing around using social networks as the accelerator.

STELTER: Now, staffers at social media giants are doing some soul-searching.

These fake sites are easy to set up and profitable for the creators. Every time we click and share, they make more money, but we are worse off.

Now, Facebook and Google are banning fake sites from making money off their ad networks. It's a first effort to choke off some of the revenue.

The bigger challenge? Providing more detection tools without threatening free speech.

GILLMOR: Suddenly, they have these, I think, these social, societal duties to help us not be faked out all the time. And yet, I don't want the terms of service of one company or two or three companies to have more influence than the First Amendment.

STELTER: The root problem is that some people want to believe the lies. That's why the responsibility isn't just Facebook or Google or Twitter's.

We all have to get a little smarter about what we share.

GILLMOR: We have to be relentlessly skeptical of absolutely everything.

We have to go outside of our personal comfort zones, and read and watch and listen to things that are bound to make our blood boil.