Interesting comparison of apps vs platforms from Ben Thompson:
An app can afford to be prescriptive about the user experience and means of interaction; in fact, the best apps have a point of view on how the user ought to use their service.
Platforms, on the other hand, are just that: a stage for actors (i.e. apps) of the user’s choosing to create a wholly unique experience that is particular for every individual user.
It follows, of course, that no successful platform can be built on advertising. Advertising demands eyeballs; platform success demands the ability to fade into the background as said unique experiences take center stage.