Note that the truly large-scale filesystems do not use the “live system + periodic backup” paradigm, but instead maintain multiple replicas of all data all the time on multiple drives, so if a drive fails there is no delay to switch to another replica.
I believe that dropbox is backed by Amazon’s S3, which is such a filesystem. Places like Amazon “eat their own dogfood” in the sense that Amazon.com itself is hosted on the same tech as is offered in their cloud – they did a lot of work to make Amazon.com reliable and durable, and then marketed the technology to do that to other companies. In that sense the tech is “proven” to the extent of Amazon.com’s stability.