Elon Musk, the world’s richest person, is fighting to turn around Twitter. Rolling out cost-cutting measures, he warned staff that “bankruptcy isn’t out of the question”. He also tweeted an old joke with a new twist: “How do you make a small fortune in social media? Start out with a large one.” Musk has a large fortune today, and Twitter is just a fragment of that. But just about two years ago, he probably couldn’t have bought Twitter.