The heads of two leading social media companies have very different ideas about paid advertisements supporting political candidates. On Wednesday, Twitter’s Chief Executive Officer, Jack Dorsey, said that the social media company would stop all political advertising. On the same day, Facebook CEO Mark Zuckerberg again defended his company’s policy of permitting political advertisements. Zuckerberg faced questions on the subject from lawmakers on the House Financial Services Committee last week. In a series of tweets, Dorsey said Wednesday that internet advertising is effective for advertisers. But he warned: “That power brings significant risks to politics.” Dorsey went on to say that his company believed that “political message reach should be earned not bought.”