The social network will feature this new tool at the top of people's News Feeds for the next few days.
Facebook has introduced a new educational tool to help users spot fake news stories posted to the social networking site.
Facebook believes the measures it has taken so far are already having an effect - Mr Mosseri says false news is less in evidence in the United States than it was during the election campaign and in mainland Europe there has not been any growth in the phenomenon.
The guidelines include looking closely at website addresses to see if they are trying to spoof real news sites, and checking websites' "about" sections for more information.
Adam Mosseri, the social network's News Feed boss, said: "We know people want to see accurate information on Facebook - and so do we".
The new feature is part of a broader plan by Facebook to clamp down false news stories, which gained outsized attention in the months leading up to the 2016 US presidential election. "It's like they have become citizens". And once they are identified, Facebook is trying to dry up the "economic incentives" of false news sites by making it hard for them to buy ads on Facebook. The company recently announced that it had partnered with third-party fact-checking organizations like First Draft to help identify false news links, which are then flagged in the News Feed. "Until Facebook finds ways to curb the folks who are generating false news and misinformation, steps like this are really only window-dressing".
With a federal election looming in Germany, Facebook is beginning to attach warning labels to made-up stories seen by German users of the web site. And that has allowed Facebook time to think more carefully about how it can tackle the issue in Canada, he said.
"We found that (young people) were quite likely to try and figure out whether something was legitimate or not when they were doing school work", said Matthew Johnson, the director of education at MediaSmarts, a not-for-profit with a self-prescribed mandate of educating young people about digital media.
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