Microsoft has sold plenty of licenses for Windows 8. Six months after its launch, the company announced that 100 million licenses had been sold, with each one representing an upgrade copy or a PC that had rolled off the assembly line and was ready for sale. A rough extrapolation from current usage share figures collected by NetMarketShare and StatCounter suggests that 60-70 million of those PCs are in use today.

But how many people are actually trying out the signature feature of Windows 8, its new apps? And how many are continuing to use those apps on a daily basis?

Microsoft collects usage data through its opt-in Customer Experience Improvement Program. So far, however, it’s only disclosed the number of apps available in the Windows Store.

That leaves the data collection and analysis to third parties. A new report from Soluto. “Windows 8 Metro Apps Usage,” suggests that the new apps are gaining traction, albeit slowly. And the most engaging apps, the ones that people download, install, and use regularly, are coming from some surprising sources. The number-one app on the list? Yahoo Mail.

Soluto collects details from users of its PC management software, which is primarily aimed at IT pros who support small businesses and consumers. The data is collected from a management agent that runs on client computers. (If you manage PCs for family members, you should try the free version.)

For this report, the company analyzed roughly one month’s worth of data from 10,848 Windows 8 machines, including desktop PCs, laptops (with and without touchscreens), and tablets. The sample population included 200 separate models from (in alphabetical order) Acer, Apple, Asus, Dell, HP, Lenovo, Samsung, and Toshiba.

A few highlights from the report:

The survey recorded 9,634 unique Metro style Windows 8 apps that had been launched at least once.

Owners of touchscreen-enabled laptops are 47% more likely to use Windows 8 apps than those with a standard laptop/notebook.

Not surprisingly, tablet owners launch nearly twice as many Windows 8 apps as do their counterparts on desktop PCs. More surprising is the finding that nearly 40 percent of desktop users launch at least one Windows 8 app every day

Among the built-in Windows 8 apps, the Windows Communications Apps (a single package that includes Mail, People, Messaging, and Calendar) was launched at least once by more than 85 percent of all Windows 8 PC owners. The Photos, Reader, and Camera apps were launched at least once by 44 percent, 27 percent, and 13 percent of Windows 8 users, respectively. The remaining Microsoft-written apps were largely ignored by the owners of Windows 8 PCs.

Among third-party apps, only Netflix made the top 10. Its app was downloaded and installed from the Windows Store and used at least once by roughly 8 percent of the sample population. Google Search was the second most popular non-Microsoft app, with slightly less than 4 percent of the sample trying it at least once.

The most interesting data in the report measures the level of engagement on the part of app users. For this statistic, Soluto starts by counting the number of Windows 8 users in its sample who have launched an app at least once, and then calculates the average number of times that app is used per week. That methodology tends to punish Microsoft’s built-in apps, which are preinstalled on every Windows 8 PC and in some cases are set as the default app for common file types: Photos for JPEGs and Reader for PDFs, for example. A PC owner is more likely to stick with an app that he or she deliberately downloaded and installed from the Windows Store.

With that caveat in mind, it’s still startling to see which Windows 8 apps are used most often.

Yahoo Mail? Yes, indeed. Hardcore techies might scoff, but Yahoo has hundreds of millions of loyal mail users worldwide, and its Windows 8 app, released in December, was built with those mainstream users in mind.

Yahoo’s app snubs Microsoft’s user experience guidelines for Metro style apps, with a large logo and a command bar along the top of the screen. When you’re reading a message, there are prominent buttons to reply to a message, delete it, or search your mailbox. Microsoft’s built-in apps, by contrast, force the user to find the hidden search charm, and most other options are hidden on a command bar that only appears when you swipe from the bottom of the screen or right-click.

You won’t find the second and third items on the list in the Windows Store unless you own a PC built by Acer or its Gateway subsidiary. These apps are preinstalled on Acer PCs, but weren’t on my machine. I downloaded and installed the Social Jogger app on an Acer touchscreen PC. Here’s a shot of the app in action.

One secret of this app’s success is that some of the Gateway PCs on which it’s installed include a dedicated key for accessing the app directly. But it’s also worth noting that the app contains prominent branding and four action buttons that are always visible in the upper right portion of the app. Again, making the app’s commands discoverable and not hiding them appears to have paid off for Acer.

Number four on the list is Lync MX, the Windows 8 version of Microsoft’s unified communication suite. Although there’s a more powerful desktop version of Lync, the Windows 8 release is easier to set up and much easier to use. And not coincidentally, it’s also incredibly touch-friendly, with an always-visible search button and easy commands for placing calls to other Lync users.

In fact, if there’s a single takeaway from this report, it should be a call to action to Windows 8 app developers: Don’t be afraid to break the Metro rules and put common commands right on the app screen. Your customers will thank you. More importantly, they’ll come back.