Quietly and inexorably, the “seven sisters” of the global technology industry are acquiring monster market values. Apple, Alphabet, Microsoft, Amazon and Facebook are the five most valuable firms in the world, and Tencent and Alibaba, their peers in China, aren’t far behind. In total the seven companies are now worth an incredible $3.6trn, 40% more than a year ago and more than the combined value of all firms with a primary listing in Britain, the world’s fourth-largest base for public companies, according to Bloomberg. Bulls argue that the tech platforms’ huge increase in value reflects their rising profits. But their share prices have moved up faster than their earnings. As a result, together they trade on a price-earnings multiple of 34 times, up from 26 times a year ago. That is not quite bubble territory, but it is getting close.