I am stuck at how to classify Dropbox and other cloud storage services within the ISP (IaaS,PaaS,SaaS) model. At their core, such services provide a cloud-backed file system which can be read and written to from any location as long as there's a connection.
A file system is sort of Infrastructure in that it's an essential resource for computing, but IaaS seems to be more about providing virtual machines and such.
It might be classified as a Platform, as in it's a specialized service which can be (and is) used to add common functionality (synchronization and unification of user data in this case) to applications. On the other hand, it also can be and is used as an end-user application/service.
Finally, we might argue that since services like Dropbox are end-user services, they are thus SaaS. But SaaS is defined by the application being online and used via a thin client, while most cloud storage providers use native software to do the syncing.
So, what is the proper classification for such products?