Between 2010 Q2 and 2011 Q2 Microsoft's marketshare in sales of new smartphones went from 4,9% to 1,6%. (According to Gartner)

That's a drop to one third during the period of Windows Mobile to WP7 transition. Pretty catastropic considering how bad WM was and how WP7 was supposed to save MS. Of course, there are few people who think that sticking a Nokia logo on WP7 phone is somehow going to turn water into wine.