Mark Cuban is one of America's most successful businessmen. He owns the Dallas Mavericks of the NBA, The AXS TV cable network, and Landmark Theaters. He's also one of the investors on ABC TV's "Shark Tank." It stands to reason, he knows more than a little about media. Facebook, Twitter, Instagram, and similar social media outlets have enabled huge numbers of people to become amateur "broadcasters." The vast majority of people have little or no training in the use of social media. Mr. Cuban says most people are making very dangerous mistakes...