Here's why the stats posted by Elimentals are relevant despite using percentages:
In the period he mentioned, the cellphone industry
grew by 71%
Since Elop announced his Microsoft strategy, Nokia market share crashed to 12% its sales declined by 31% all while the industry had an explosive year of growth of 71%.
For any company or platform to be showing shrinking market share in such an environment points to abysmal failure.
Notice that the only platforms that are showing failure are Symbian, Blackberry and Windows.
Now an even more important part of these stats when considering development is:
If I have to spend x to develop for a platform, I would like to know there is a significant enough user base for me to have any hope of gaining revenue from said platform's user base.
If platforms 1 and 2 have significant market share and are growing, those are safe bets for my invested money to grow.
If platforms 3, 4 and 5 are shrinking relative to the rest of the market, those are not worthwhile to spend money on when its likely that even if I defy all odds and get 100% of that platforms users to buy my app, I will probably not make back the money I put into development.
Now factor in that on WP7 the language to be used is C# while on all other platforms I can develop from a common codebase on C, C++ or even QT if I wanted to target multiple platforms cost effectively.
This makes it even more costly to target WP7 as I have to now rewrite a significant portion of code
just to target one platform with shrinking market share.