I subscribe to Oyster, a service that allows me to read as many books as I please from a pretty decent selection.  In its catalog are a large selection of self published titles by authors trying to get exposure for their work.  Oyster (or any other subscription service seeking to boost subscriber engagement) could simply create a third party cookie that indicated whether a user was logged into their service.  Then ad networks could show service-specific ads.

An example: William has published, “Space Weasels and Blue Food”, a book of science fiction short stories on Oyster (via SmashWords).  He wants to make a name for himself so he creates an ad for his book on Facebook that, because of the third party cookie, is only shown to subscribers of Oyster that happen to be logged in.  Users simply click the ad and can immediately be enjoying William’s book.

This could work on any subscription service for any type of media.  For mobile apps I’m not sure it would work but should work on mobile websites.