I wonder if they still keep everything on Amazon S3. I'm sure most users will not fill up the Terabyte, but still S3 costs 330USD per Terabyte per year (not including costs for outbound traffic) (http://aws.amazon.com/s3/pricing/).
While some would argue dropbox to built their own DC. I still dont see it as a viable solution. Google, Apple, and May be Amazon dont need to make money from these Storage. And all 3 gets to leverage their existing infrastructure.
Dropbox, however, is a one trick pony. Expect them to stick around as a premium priced service. Until someone acquires them and ruins everything...
I trust that iCloud, in some iteration, will be around for as long as people are buying iMacs and iPhones.
Microsoft, miraculously, has been changing their spots lately, but I expect Windows Live Folders – I mean, Windows Live Skydive – or wait, just Skydive – er, I mean Onedrive – will be around in some iteration. What that will mean, though, is another question. They killed off Games for Windows Live pretty unceremoniously, and now Dark Souls, among other games, just straight up won't work properly. You can't save your game if you aren't connected to Games for Windows Live, which you can't do, retroactively crippling third party games.
So for throwing my documents onto so that they still exist after I wipe my Windows machine for the Nth time, Skydive is great, but I don't yet trust them to change direction/focus and ruin any plans businesses have made around them.
Google… well, they've cancelled enough stuff that I can't trust them with anything important related to business or lifestyle both.
The biggest difference with Backblaze would seem to be that their service can easily assume minimal reads; besides the backup index, the vast majority of their disk usage is probably one-time writes, plus the occasional massive bulk read.