I'm not too familiar with Admob or other mobile ad platforms, but don't they let you sell inventory directly already? I think the reason this is less desirable for both publishers and advertisers is the amount of work involved in direct sales efforts. As an app publisher, you may not have time to go out and sell direct spots on your network (which will be necessary if you don't already have the kind of data about your users that facebook / google provide). Advertisers have a similar problem on the other side, in that using facebook/google services allows them to easily target their audience based on data about them. How will your adnetwork address that need?