"Breaking news: Mark Zuckerberg and Facebook just endorsed Donald Trump for re-election," Warren's sponsored post reads.

"You're probably shocked, and you might be thinking, "how could this possibly be true?"

"Well, it's not. (Sorry)."

Warren was highlighting an advertising loophole that means politicians are exempt from the company's third-party fact-checking program.

"Facebook holds incredible power to affect elections and our national debate," she told her 3.3 million Twitter followers.

"They've decided to let political figures lie to you - even about Facebook itself - while their executives and their investors get even richer off the ads containing these lies.

"Once again, we're seeing Facebook throw its hands up to battling misinformation in the political discourse, because when profit comes up against protecting democracy, Facebook chooses profit.

"Facebook already helped elect Donald Trump once through negligence.

"Now, they've changed their policy so they can profit from lies to the American people. It's time to hold Mark Zuckerberg accountable."

The decision to not fact check political adverts was announced by Sir Nick Clegg, former British deputy prime minister and now Facebook's vice president of global affairs.

Speaking at the Atlantic Festival in Washington DC last month, Clegg said "it is not our role to intervene when politicians speak".

"We do not submit speeches by politicians to our independent fact checkers and we generally allow it on the platform even when it would otherwise breach our normal content rules," he said.

Australian Associated Press