Shares of Twitter Inc. (TWTR) - Get Report fell in after-hours trading after the company announced it is banning all political ads.

The new policy was revealed in a series of tweets from the social media titan's CEO Jack Dorsey.

"We've made the decision to stop all political advertising on Twitter globally," he wrote. "We believe political message reach should be earned, not bought."

Dorsey offered several reasons for the policy change. "While internet advertising is incredibly powerful and very effective for commercial advertisers, that power brings significant risks to politics, where it can be used to influence votes to affect the lives of millions," he said.

"Internet political ads present entirely new challenges to civic discourse: machine learning-based optimization of messaging and micro-targeting, unchecked misleading information, and deep fakes. All at increasing velocity, sophistication, and overwhelming scale."

Dorsey's stance offers a powerful contrast to Facebook (FB) - Get Report  CEO Mark Zuckerberg, who said earlier this month that his company won't fact-check ads by politicians or their campaigns, enabling them to perpetrate any untruths they please. Facebook has taken a lot of heat for that decision.

One of Dorsey's tweets may represent a not-so-veiled jab at its social media competitor. "It's not credible for us to say: 'We're working hard to stop people from gaming our systems to spread misleading info, buuut if someone pays us to target and force people to see their political ad...well...they can say whatever they want!" he wrote. 

Twitter's new policy begins Nov. 22.

Twitter shares stood at $29.52 in after-hours trading, down 1.2%.

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The author of this story owns shares of Facebook and in the past owned shares of Twitter.