Facebook is not a lover of windows phone or any other operating system , it is a company that seeks profit.....from a business point of view...if I would have to release a new app whose success is under question and which needs to grow , I will definitely try it on one with the bigger market so that of once the app proves to be a flop then my investment in the app will not be lost completely......what if the app doesn't gets as much attention by the users..? The whole investment of time and money will be lost.....so that's why I believe that fb will release the slingshot app but after it gets some hype over more popular operating systems...