NEW YORK: As some observers argue that fake news sites may have affected the result of the US presidential election, Google and Facebook have said they are taking steps to bar such sources from using their ad networks.

Earlier this month, for example, Buzzfeed reported that Macedonian teenagers had set up "at least 140" US politics websites that were pumping out fabricated pro-Trump content simply to generate traffic that would earn them money from advertising.

The apparent ease with which this could be done highlights once again the issue of digital ad fraud, which the ANA earlier this year estimated was worth more than $7bn annually, although much of that was said to be driven by non-human traffic.

"Moving forward, we will restrict ad serving on pages that misrepresent, misstate, or conceal information about the publisher, the publisher's content, or the primary purpose of the web property," a Google spokeswoman told Reuters.

Similarly, Facebook said that fake news sites would be prevented from using Facebook Audience Network to generate ad revenues.

Earlier, Facebook CEO Mark Zuckerberg rejected as a "crazy idea" the notion that fake news on the social networking platform could have influenced the outcome of the election, and claimed that 99% of what people see is authentic.

"The hoaxes that do exist are not limited to one partisan view, or even to politics," he said in a Facebook post. "Overall, this makes it extremely unlikely hoaxes changed the outcome of this election in one direction or the other."

But not everyone at Facebook agrees with that assessment, with one staffer telling Buzzfeed that "fake news ran wild on our platform during the entire campaign season".

A group of disaffected employees are reported to be meeting in secret and reviewing whether the platform had responded adequately to user reports of fake news and whether features that scan for such content had been effectively used.

Data sourced from Reuters, Facebook, Wall Street Journal, Fortune, Buzzfeed; additional content by Warc staff