Reportedly around two thirds of mobile apps are never even downloaded.[1] And of those that are downloaded, a majority are used fewer than 10 times in total.[2] It's not the number of apps that matters, but the number of apps that people will actually use. In this regard, both the Android and iOS app ecosystem are still relatively immature and poised for considerable growth.
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[1] http://gigaom.com/europe/app-store-infested-with-zombie-soft...
[2] http://gigaom2.files.wordpress.com/2012/06/localytics_app_us...