SAN FRANCISCO (AP) — Twitter, reacting to growing concern about misinformation spread on social media, is banning all political advertising from its service. Its move strikes a sharp contrast with Facebook, which continues to defend running paid political ads, even false ones, as a free speech priority.

“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,” Twitter CEO Jack Dorsey said Wednesday in a series of tweets announcing the new policy.

Facebook has taken fire since it reiterated in September that it will not fact-check ads by politicians or their campaigns, which could allow them to lie freely. CEO Mark Zuckerberg told Congress in October that politicians have the right to free speech on Facebook.

Zuckerberg wasted no time responding to Twitter’s move. During Facebook’s conference call for earnings, which began less than an hour after Dorsey’s tweet, the Facebook chief offered an impassioned monologue about what he called his company’s deep belief “that political speech is important.”

Zuckerberg stood by the company’s decision to run unchecked political ads and denied that the choice is financially motivated, saying such ads make up less than half of a percent of Facebook revenue.