In this month's look at Windows Phone handset usage from AdDuplex, the Lumia 520 (and, in the US, the Lumia 521) continues to surge, and cement its lead over competing devices. Indeed, the Lumia 520 now represents over 20 percent of all Windows Phone handsets currently in use.

As you may remember, AdDuplex bills itself as the largest cross-promotion network for Windows Phone and Windows 8 apps, empowering developers and publishers to promote their apps for free by helping each other. And each month it provides a tantalizing glimpse at which Windows Phone (and Windows) devices people are actually using.

Some trends from this month's report include:

Lumia 520. In two months, the Lumia 520 has surged from 13 percent of all Windows Phone usage to 18 percent and now, this month, to 21.3 percent. Looking at just Windows Phone 8 handsets, the Lumia 520 is an amazing 31.6 percent of all devices. This is no coincidence: As you can read in my posts In Praise of the Nokia Lumia 520 and Lumia 520/521: Meet the New Normal, this device really is something special, the perfect combination of super low pricing ($99, no contract) and functionality.

Lumia 521. The basically identical Lumia 521—which is available on T-Mobile and MetroPCS in the United States—is likewise surging in this country and in fact almost doubled its usage month-over-month. According to AdDuplex, the Lumia 521 is the second most-used Windows Phone handset in the US with 14.2 percent share, up from just 8 percent last month. That means it's about to overtake the current number one device, the Lumia 822, which has 14.6 percent share (and is exclusive to Verizon). Message received: Low cost excellence does sell well.

It's all Lumia. Once again, 9 of the top 10 phones are Nokia Lumias: The Lumia 520, 920, 620, 710, 800, 820, 720, 610 and 900 once again fill out the top of the list, with the HTC 8X again in 10th place, though falling to just 3.2 percent usage.

Lumia 1020. The greatest smart phone ever made hasn't proven a big hit with consumers, from what I can tell. This month, the Lumia 1020 is the 28th most-often-used Windows Phone device overall, compared to its 32nd place finish last month. If you just look at Windows Phone 8 handsets, it's unchanged at number 16. And in the US, Lumia 1020 is number 11 overall.

Nokia controls almost 90 percent of the Windows Phone market. The world’s biggest maker of Windows Phone handsets is increasingly the world’s only maker of Windows Phone handsets. Nokia now accounts for 88.4 percent of all Windows Phone usage worldwide, up from 87 percent last month. 90 percent usage share is only a matter of time.

Windows Phone 8 vs. Windows Phone 7.x. 67 percent of the Windows Phone handsets in use worldwide are running the latest OS version, while 33 percent are still using Windows Phone 7.x, a change of two percentage points for each.

GDR2. The latest version of Windows Phone 8 is rolling out at a very fast clip, bursting expectations: Fully 30.6 percent of all Windows Phone 8 handsets in use have GDR2, up from just 3 percent last month. Nokia says that it will be available to all Lumia users by the end of September.