Thought exercise: Facebook ad network driven by users

Here's a thought exercise:

Why doesn't someone build, as an app, a Facebook ad network.

Facebook sells advertising that puts ads on our pages and in our newsfeeds.   Could you build an ad app that people would ad that could become like AdSense for Facebook (or, in fact, an actual AdSense for Facebook app).  Conceivably, it could use some of the data of the signed in user to tip off the engine to get more relevent results, and you could pay the person who added the app.  Imagine that, an advertising network that never has to pay a dime for the platform itself...  you wouldn't even need to cut Facebook in.

Of course, there would be downsides.   People might not like you selling stuff within their feeds and bringing a commercial feel to a social environment, so they may turn you off in their feeds... or join you and ad the app themselves so they can make money, too.

I would imagine that if such a thing was successful, Facebook would adjust its policy on ads in apps.