Earlier quoted context omitted.
Pay for exposure already exists within the app store. Large companies pay around $10-$20K a day depending on the category on crap ad networks (won't name them) that incentivize the users to download the companies app in order to "get to the next level" or something like that. The downloads are incredibly cheap and you get thousands and thousands of them. The app store ranking is almost entirely based on download velo…
Facebook just recently started doing this as well. You can now pay to get your app promoted in people's news feed.
I've had a lot of success on FB mobile install ads.