The info comes from Vision Mobile, which has gathered data from about 10.000 developers in 137 countries around the world. According to the report, 70% of developers target Android for their apps, while 51% target iOS, which would logically follow that there are a lot of Android exclusives, even if it doesn't always feel that way. Windows Phone has a respectable amount of attention from developers at 28%. And, unsurprisingly, BlackBerry brings up the rear at 11%, which puts the platform behind even HTML5 development (15%).
Of course, the aim is to make money, and the report says that isn't all that easy anymore. Basically, the mobile app world has become so saturated that it is very difficult to generate significant revenue. Although, depending on your aims, you can still generate a nice bit of additional income. Almost 25% of all app developers across platforms make no money at all. 64% of Android developers make less than $500 per app per month, and 50% of iOS developers are under that threshold as well. iOS is still the way to go, if you're not looking to quit your day job, because only about 33% of devs on the platform make less than $100 per app per month, while that number is at 50% on Android, and even higher on WP and BB.
The real money is in aiming at the enterprise sphere rather than consumer, where apps are much more likely to make serious bank. But, it takes a lot more work and investment to build a successful enterprise app. It's that old risk/reward quagmire that can be tricky to navigate.