[chickenbane]

Dropbox is currently indispensable for iOS users. Most iOS people I know use Dropbox as the clunky-but-works app-to-app sharing mechanism. Dropbox is not as popular on Android because apps can integrate more naturally.

iCloud may be easier and is the probable successor, but Apple has only recently gotten its cloud game together and its free option is only 5GB.

Eventually the platforms are going to make using a third party obsolete (arguably already so on Android) so I predict we're at peak Dropbox usage. Especially considering Google's new Pixel phones will store all photos at full resolution for free, I'm sure Apple will at least have to increase their free storage tier.

[0xffff2]

I'm confused by your comment... I'm a recent convert to iOS from Windows Phone, and I've just kept on using OneDrive for my documents. I'm not even sure what "app-to-app sharing" means. Dropbox definitely isn't indispensable to me.

[kuschku]

For example, creating a file in one app, editing it in another app, sending it to a third app to upload somewhere else.

That's hard to do without a filesystem or proper Intents.

iOS has neither, so people use Dropbox as filesystem.