Cloud file storage is also available via Chwala/iRony components of Kolab with the capability to integrate various storage backends. Files are accessible via storage layer access options, WebDAV and Kolab web interface integrating Roundcube.
Tarsnap is not recommended on PRISM Break due to its strict copyright on the client that makes it difficult to replace the service in the event Tarsnap is shut down.
BitTorrent Sync, MEGA, and SpiderOak are services that are built on either partially or fully proprietary software. They will not be recommended on PRISM Break until they open source the entirety of their codebase.
With closed source software, you need to have 100% trust in the vendor because there's nothing except for their morality in the way of them leaking your personal information. Even if you can vouch for their integrity, proprietary software invariably has more uncaught security bugs and exploits because there are fewer eyes examining the source code.
Another alternative to cloud storage is local backup with external hard drives and USB flash drives. This method is reliably more secure than storing data on a network, but comes at a convenience cost.