Continuing its monthly look at Windows Phone usage, mobile ad firm AdDuplex will this week report that Windows Phone 8 usage has slowed considerably, while the first Windows Phone 7.8 devices are starting to appear in its findings.

I previously wrote about AdDuplex’s usage data in Report: Windows Phone 8 Usage Surges, last month, and in Interesting Windows Phone Stats the month before.

This month, Windows Phone 8 has risen to 26 percent of all Windows Phone handsets. But that’s a pretty small leap over the previous month, when it was 19 percent. This could be troubling news if you’re a fan of the platform.

On the flipside, usage of Windows Phone 7.8 is up: 16 percent of all Windows Phone 7.x handsets are now running the latest (and last) Windows Phone 7.x update. 84 percent are still running Windows Phone 7.5 or older. And over half of all Windows Phone 8 users are already on the hastily released “Portico” revision.

Of the top five most frequently-used Windows Phone handsets, only one comes with Windows Phone 8, and that device, the Lumia 920, represents just 10 percent of all Windows Phone usage. The Lumia 820, in 6th place, is used by 5 percent of users. And HTC racked up 7th and 8th place, with the 8X and 8S, respectively garnering 4 percent and 3 percent of overall usage.

The top five most frequently-used Windows Phones are all Nokia devices: The Lumia 800 (18 percent), the Lumia 710 (17 percent), Lumia 610 (16 percent), Lumia 920 (10 percent) and Lumia 900 (6 percent). Overall, Nokia devices represent about 78 percent of all Windows Phones in use, AdDuplex claims.

Oddly, HTC is the second-biggest maker of Windows Phone handsets now, and I have to think the 8X has a lot to do with that. 13.7 percent of all Windows Phones in use are made by HTC, compared to just 6 percent for Samsung.