There is no social media platform more important to the state of contemporary journalism than Twitter. It’s where journalists — and news junkies in need of a fix — disproportionately choose to spend their time. So, a change at the top of Twitter is significant news. This week, Twitter's CEO, Jack Dorsey, made the sudden public announcement that he had quit as CEO of Twitter. The surprise move ends Dorsey’s much criticised tenure as chief executive of both Twitter and Square, his digital payments company, which led to...