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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 story first comes out, not three days later. That's the case with both true news and fake news.

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.

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