Microsoft's data source and compiled list is unfortunately not available:
That sounds pretty solid, especially given how few apps most people use on a regular basis. But among the four missing is Instagram (the others are Pinterest, Wells Fargo and Voxer), which could be the sort of killer app that would prevent people from making the jump to Windows Phone... But how exactly did Microsoft arrive at its top-50 list? The company wouldn’t share its list with us, and Belfiore only explained that a Windows Phone team compiled the list based on download rates and usage on apps available for iPhone and Android smartphones.
Not to be put off the task, Chang headed over to Distimo to build her own list, built up of the top 25 free apps and paid-for apps on both iOS and Android to come up with a 100 strong list.
Overall, things look promising. Of the 91 apps on the lists, Windows Phone already has or will have 32 of those apps. There are very similar alternatives for 16 apps, and six are not relevant or necessary to the platform. Thirty-seven apps are not available on Windows Phone, though the majority are new games or apps that are only available on one platform (iPhone or Android).
One big difference between this list and Microsoft's? Wired have posted their list!
Chang's full post can be found at Wired's Gadget Lab.