In September, President Trump’s re-election campaign released an ad that included an incorrect statement about Mr. Biden’s dealings with Ukraine. The campaign posted the ad on Facebook and the president’s Twitter account. Between the two services, the ad has been viewed more than eight million times.

Mr. Biden’s campaign publicized letters that it had written to Facebook, Twitter, YouTube and Fox News, asking the companies to ban the ad. But it remained up. In mid-November, the Biden campaign released a website called Just the Facts, Folks.

Jamal Brown, a spokesman for the Biden campaign, said it was not the campaign’s responsibility alone to push back on all falsehoods. But, he said, “it is incumbent upon all of us, both public- and private-sector companies, users, and elected officials and leaders, to be more vigilant in the kinds of content we engage and reshare on social media.”

Several months ago, a team at the Democratic Congressional Campaign Committee flagged some ads on Facebook to the office of Representative Ilhan Omar, a Minnesota Democrat. The ads called for an investigation into unfounded accusations that she had violated several laws.

After the committee and Ms. Omar’s campaign contacted Facebook, the company said it would limit the prominence of the ads in people’s feeds. But the ads, which have now reached over one million views, remain active.

Facebook does not remove false news, though it does label some stories as false through a partnership with several fact-checking organizations. It has said politicians like Mr. Trump can run ads that feature their “own claim or statement — even if the substance of that claim has been debunked elsewhere.”

In October, Twitter announced plans to forbid all political ads. But the company does not screen for false accusations. Twitter said it did not want to set a precedent for deciding what is and is not truthful online.