The days when Mark Zuckerberg felt like he could be glib about the idea of Russia using Facebook to interfere in the presidential election suddenly seem long ago.

On Thursday, facing mounting pressure on his company from Congress, the Justice Department and others, a very serious-looking Zuckerberg took to Facebook Live and pledged to take a range of actions to make sure future elections, in the U.S. and elsewhere, will be safe from clandestine meddling.

"The integrity of our elections is fundamental to democracy around the world," he said. "I don't want anyone to use our tools to undermine democracy. That's not what we stand for."

By "our tools," Zuckerberg was referring in part to so-called "dark posts," which allow advertisers to place vanishing ads into users' News Feeds without hosting them on the advertiser's own page. Dark posts make it possible to run ads that are effectively invisible to everyone but the recipient.

Thanks to lobbying by Facebook and others, political ads on social media are already not subject to the same disclosure requirements as ads that run on TV or in print. Combined with dark posts, that makes it possible to target Facebook users with false or contradictory ads that can't be fact-checked or traced back to their sources.

Dark posts will no longer be allowed for political ads. "Not only will you have to disclose which page paid for an ad, but we will also make it so you can visit an advertiser's page and see the ads they're currently running to any audience on Facebook," Zuckerberg said, calling this "perhaps the most important step we're taking." Implying that Facebook will encourage other internet companies to adopt tighter standards, he touted "a new standard for transparency in online political ads."

Eliminating dark posts was one of nine steps Zuckerberg outlined to safeguard future elections. Among the others: Facebook is strengthening its process for reviewing ads, increasing the size of its election-integrity and security teams, and partnering with election commissions around the world. The company has also reached an agreement to provide some 3,000 ads that have been traced to a Russian "troll farm" to Congressional investigators looking into Russian activities.

"This has been a difficult decision," wrote Facebook's general counsel, Colin Stretch, of handing over the ads. Facebook had said previously its user-privacy policies prevented it from turning over content.

But with each new revelation about the scope of Russia's propaganda efforts, pressure on Facebook to stop circling its wagons and appear to take foreign interference seriously has ratcheted up. A recent investigation showing Facebook allowed advertisers to target messages to users who had espoused anti-Semitic views didn't help.

Whereas once it seemed like Facebook could ride out any bad publicity on the strength of its inexorable growth as a business and cultural force, it's increasingly operating in an environment where billionaire technology moguls like Michael Bloomberg and Pierre Omidyar are publicly criticizing it and even calling for it to be regulated.

Zuckerberg is clearly feeling the heat, and anxious not to be judged too harshly. "It's a new challenge," he said, "for online communities to have to deal with nation-states attempting to subvert elections."