Twitter has decided to ban all political advertising, citing the risk of spreading lies and its impact on democracy.

The social network made the announcement just minutes before Facebook CEO Mark Zuckerberg talked to analysts about his company's Q3 2019 earnings.

On the conference call, Zuckerberg strongly defended political ads, and framed it as a free speech issue.

But critics, including Twitter CEO Jack Dorsey, argue that banning political ads isn't censorship as politicians are still free to post on social networks normally.

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Twitter CEO Jack Dorsey just threw down the gauntlet to Mark Zuckerberg over political ads — but the Facebook CEO isn't backing down.

On Wednesday afternoon, Dorsey announced that his social network is banning all political ads, an area of online advertising that is growing increasingly controversial amid rules from tech companies that allow politicians to pay to spread deliberate lies.

The Twitter chief executive said in a series of tweets that "political message reach should be earned, not bought," prompting praise from some Democratic politicians in the US — along with criticism from President Donald Trump's re-election campaign. Facebook has also been under fire about its refusal to fact-check or rein in political ads on its platform — but Zuckerberg has made clear that he doesn't intend to budge.

Just minutes after Dorsey's announcement, Zuckerberg appeared on a conference call with financial analysts to discuss his company's Q3 2019 earnings. He kicked it off with a monologue about "principles" and free expression, in which he strongly defended Facebook's stance.

"Right now, the content debate is about political ads. Should we block political ads with false statements?" said the 34-year-old billionaire chief executive. "Should we block all political ads? Google, YouTube and most internet platforms run these same ads, most cable networks run these same ads, and of course national broadcasters are required by law to run them by FCC regulations. I think there are good reasons for this."

"In a democracy, I don't think it's right for private companies to censor politicians or the news. And although I've considered whether we should not carry these ads in the past, and I'll continue to do so, on balance so far I've thought we should continue. Ads can be an important part of voice — especially for candidates and advocacy groups the media might not otherwise cover so they can get their message into debates," he said.

He added: "And it's hard to define where to draw the line. Would we really block ads for important political issues like climate change or women's empowerment? Instead, I believe the better approach is to work to increase transparency. "

Less than 0.5% of Facebook's revenue in 2020 will come from ads from politicians, Zuckerberg said. You can read his full speech on his Facebook page.

Zuckerberg's remarks, which were scripted in advance of the call — and very likely in advance of Twitter's announcement — indicate that Facebook does not currently intend to back down on this issue, and that it intends to frame its stance as a matter of free speech.

However, critics argue this is the wrong way to look at it, and that deciding not to let politicians run ads does not amount to censorship as they can still post freely on the platform.

"A political message earns reach when people decide to follow an account or retweet. Paying for reach removes that decision, forcing highly optimized and targeted political messages on people. We believe this decision should not be compromised by money," Dorsey wrote in a string of tweets.

"This isn't about free expression. This is about paying for reach. And paying to increase the reach of political speech has significant ramifications that today's democratic infrastructure may not be prepared to handle. It's worth stepping back in order to address."

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