A few weeks ago, Facebook made it clear that posts shared by politicians are exempt from Facebook's community standards and also from fact-checking.

Faced with a stark real-world test, though, Facebook appears once again to be erring on the side of letting misinformation circulate far and wide if a politician promotes it.

The Trump campaign in the past week has been airing ads on Facebook making false accusations about former Vice President Joe Biden, a Democratic candidate for the 2020 presidential nomination.

The ads' claims about Biden's activities in Ukraine and elsewhere have been repeatedly debunked as baseless conspiracy theories, not only by media outlets but also by Republican politicians.

Facebook said in September that politicians' speech was exempt from the company's content rules, but that exemption had two limitations.

Previously debunked information, the company said, would not be allowed in advertisements:

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