Localytics collects its data from its app metrics service, which does mean it may be subject to sampling bias (e.g. biased towards device owners likely to download apps, so high-end devices may be over-represented). However, the Localytics service is used by a number of high profile apps, which should give it a decent sample size, and so can certainly be considered reliable enough to give a general indication of changing trends, while the precise numbers are perhaps best regarded with some caution.
The overall trend shows that Nokia devices are an increasing proportion of active Windows Phone devices, increasing from 22% in January to 59% in July. This growth has not necessarily come at the expense of the other Windows Phone manufacturers, with the number of non-Nokia devices also on the rise. Indeed the Localytics charts suggest that the number of active Windows Phone devices have more than doubled from January to July, a number that is in line with known sales figures. That said, the numbers do also suggest that the accelerating growth of Windows Phone devices is being led by Nokia, with other manufacturers maintaining, or perhaps slightly increasing their existing sales volumes.
Localytics numbers are for active Windows Phone devices. The numbers for current sales are likely to be tilted further towards Nokia. As we noted in July, in an article on Microsoft's quarterly results, back of the envelope calculations suggest Nokia could be responsible for 80% of current Windows Phone device sales.
While Microsoft did not report on Windows Phone device sales directly it did note that "Windows Phone units increased more than 50% q/q". Gartner estimated that 2.7 million Windows Phone devices were sold in Q1 2012, while IDC gave an estimate of 3.3 million for the combined figures of Windows Phone and Windows Mobile in Q1 2012. A 50% increase for these figures would be and 4.05 million 4.95 million respectively for Q2 2012, which gives a 4.5 million middle number estimate.
In its Q2 results Nokia reported that it had sold 4 million Windows Phone Lumia devices, which, together with the estimates above, would suggest Nokia is responsible for between 80% and 90% of all Windows Phone device sales. That's a statistic that underlines the importance of Nokia to Microsoft's mobile ambitions.