Facebook says it will fact-check ads posted by one political candidate in California, after previously saying it would not do so for politicians.
In September, the social network said it would not subject politicians or candidates to fact-checking.
To test the policy, Californian Adriel Hampton registered as a candidate and tried to run a misleading ad.
But Facebook blocked the ad and said Mr Hampton had "registered as a candidate to get around our policies".
Why has this happened?
In September, Facebook clarified its fact-checking policy and said it would treat all posts by politicians as "newsworthy content" that should "be seen and heard".
Critics said the move would give politicians complete freedom to create deliberately misleading advertisements that could be promoted to millions of people on the social network.
In October, Facebook's chief executive Mark Zuckerberg was questioned by US congresswoman Alexandria Ocasio-Cortez about the policy.
She asked whether she would be allowed to run ads on Facebook falsely claiming that Republican candidates had backed the "Green New Deal" environmental policy.
"I think probably," said Mr Zuckerberg.
Mr Hampton, who runs a digital media agency, was inspired to create exactly such an advertisement.
In the ad, footage of US senator Lindsey Graham was edited to look like he was speaking in support of the Green New Deal.
It was posted on the Facebook page for a political campaign group called The Really Online Lefty League (Troll).
Facebook eventually took the advert down.
Mr Hampton posted on Twitter, asking whether any US politician would run the false ad to test Facebook's policy.
He then announced he had registered to stand in an election for governor of California and would try to run a false ad himself.
But Facebook said in a statement: "This person has made clear he registered as a candidate to get around our policies, so his content, including ads, will continue to be eligible for third-party fact-checking."
Mr Hampton said: "Apparently, it's only OK to lie on Facebook if you don't tell them you're lying."
Political ad ban
On Wednesday, Twitter announced it would be banning political advertising globally.
Chief executive Jack Dorsey said the reach of political messages "should be earned, not bought".
But Conservative MP Damian Collins, who is chairman of the Digital, Culture, Media and Sport select committee investing fake news and disinformation online, said the move could have unintended consequences.
"The problem on [Twitter] has been big networks of fake bot accounts, rather than legitimate advertisers. This move could make life easier for the peddlers of fake news," he tweeted.