Microsoft has said for years that its customer base includes 1.5 billion Windows users. Among pundits and analysts, this number is often treated as authoritative and precise. It is not. It is, rather, an aspirational bit of rhetoric, called forth when Microsoft’s senior management wants to emphasize the sheer size of the Windows customer base to motivate its workforce or rev up its partners. Satya Nadella’s invocation at a Windows 10 event in January 2016 is a perfect example of the genre: The fact that there are 1.5 billion users of Windows is incredible and humbling. It’s a responsibility that none of us at Microsoft take lightly. I believe that 1.5 billion number has shrunk quite a bit in the past few years. Here, let me show you. The rise of Windows 10 Microsoft executives have, unsurprisingly, focused mostly on the growth of its Windows 10 installed base, reporting steady growth over the past five years. The company can be extremely confident about that metric, thanks to the update and telemetry components built into every copy of Windows 10. (As my colleague Mary Jo Foley notes regularly, Microsoft’s “monthly active devices” metric counts devices that have been in contact with…