My guess is that it is shrinking, not growing.
The only way to be sure would be to see sales numbers from Microsoft. The fact that Microsoft hides those numbers is telling.
Overall I think it would have been a good idea if Microsoft had done better in separating the brands. Right now its to confusing for the average user if reporters can't get it right.
Or we could look at the size of the market at times when we know the percentage of the market they have, which gives us absolute numbers, and then graph those over time.
http://www.appleinsider.com/articles/11/08/11/apples_ios_and...
Calculating from that, Microsoft has plummeted from 3,041M units to 1,724M units. In other words, Microsoft is now shipping almost half the number of phones it was shipping last year.
That's much worse than I had imagined.
Another datapoint, Bada has doubled in marketshare and now exceeds that of Microsoft.
1) WP7 + Windows Mobile is probably still growing in absolute numbers, but just being outpaced by Android and iOS growth
2) These numbers only represent consumer devices, not business; from the comScore report: "Data on mobile phone usage refers to a respondent’s primary mobile phone and does not include data related to a respondent’s secondary device.". In particular, many people have iPhone's for personal use and a BlackBerry/Windows phone for business use.
edit: formatting