A new AI tool created to help identify certain kinds of substance abuse based on a homeless youth's Facebook posts could provide homeless shelters with vital information to incorporate into each individual's case management plan. Credit: CC0 Public Domain

Facebook's effort to limit the spread of fake news using outside fact-checkers appears to be having an effect—though that finding comes with a major caveat.

Once a story receives a false rating from a fact-checker, Facebook says, subsequent "impressions" can fall off by 80 percent. Impressions count the number of times Facebook users see a particular post.

But it routinely takes more than three days for a story to receive a false rating. And most impressions occur when the first comes out, not three days later. That's the case with both true news and .

The information was shared in an email from a Facebook manager sent to the company's fact-checking partners, including The Associated Press. Facebook gave an AP reporter access to the email.