SEATTLE—At its Build developer conference, Microsoft announced that Windows 10 has now passed 500 million monthly active devices.

A little over a year ago, the company said that the operating system had reached 300 million systems.

As the operating system nears the end of its second full year on the market, it's clear that it's going to fall a long way short of the company's original estimates. At launch, the ambition was to reach 1 billion devices over the first two to three years of availability, but this estimate assumed that Windows 10 Mobile would be a going concern, selling something of the order of 50 million or more devices per year.

But that number is now approximately zero, leaving a significant gap in the expected userbase.

Nonetheless, 500 million devices is significant. Windows 10 remains the fastest-growing Windows release of all time. That growth was stimulated by the free upgrade offered to Windows 7 and 8.1 users during the first year of Windows 10's life. Surprising many observers, Microsoft did officially end that offer. Unofficially, even the latest Creators Update version of Windows 10 will successfully install and activate using a valid Windows 7 or Windows 8.1 key, so while Redmond no longer advertises the availability of the free upgrade, in practical terms, nothing has changed since day one.

The big hurdle now, as is always the case with new Windows releases, is enterprise adoption. Microsoft posted some surprise early enterprise wins, with for example, the Department of Defense being quick to certify and adopt the operating system. Certain enterprise-specific features, such as virtualization-based security, can be compelling in security-conscious enterprise environments (at least on hardware that's modern enough to support them), but corporate inertia will likely remain a drag on wider adoption.