Microsoft reports that in the last 6 months alone, there have been 1 billion downloads at the Windows Phone Store, totalling 2 billion since WP’s existence (October 2010). That’s some pretty excellent growth in the last 6 months, no?
Just for comparison, Ovi Store was launched in May 2009. About 18 months later we reported that 1 billion download ‘rate’ had been reached (November 2010) Note that’s not yet 1 billion downloads. I believe the expected 1 billion was to come after 23 months. Similarly, it was August 2011 that the cumulative 2 billion for Nokia Store was reached. Also a 6 month time frame. Considering the differences in sales between Symbian and WP, it shows that the approx 40m WP users more active at their downloading (relatively more apps/quality apps).
Apple, who popularised the app store format took about 14 months to reach 2 billion, in September 2009, with 50M devices (iPhones/iPod touches) and 85,000 apps. The first billion was reported in April 2009, about 5 months to get the second billion.
I couldn’t quite find similar details for Android.