If good comparisons and financial data are absent, such estimates are more art than science, says Brian Wieser of Group m, the world’s largest advertising buyer. That makes it even harder to put a number on Google’s advertising business as a whole (Jefferies’ estimate is $539bn). Ms Warren wants to split it into an ad marketplace and services that operate in it. But valuing its constituent parts is guesswork. The firm is not forthcoming with numbers.

The fuzziness of Ms Warren’s plan also makes estimating a total break-up value difficult. If Facebook has to part with WhatsApp, why should it keep Messenger, its other instant-messaging service? Or why should Apple keep iMessage? Both may be regarded as services on top of a platform utility. It is similarly unclear what would happen to the app stores of Apple and Google or the cloud-computing arms of Amazon and Google (and Microsoft’s, for that matter, a rival to Amazon). A spin-off of Amazon Web Services, for example, would create the world’s second-mostvaluable corporate IT firm. It would be worth $438bn, says Morgan Stanley, a bank—about four times more than IBM.

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