You're actually talking about MAID (Massive Array of Idle Disks), a technology that I first encountered in 2002. Now-bankrupt Copan Systems was the company I first encountered that was doing MAID, and New SGI (i.e. former Rackable Systems) bought their assets out of bankruptcy in 2010. Most storage companies now offer MAID add-ons for their storage arrays, though not all of them allow completely powering down the drive like Copan's solution did.
The upsides of MAID: Disks are cheap. Turning on and spinning up a hard drive to pull up some bits is faster than a robot fetching a Blu-Ray disk, placing it into a drive in the jukebox, and waiting for the disk to spin up and come online. You could store many more bytes in a cabinet with MAID than you could in an optical disk cabinet.
Downsides: The disk drives in a MAID array simply don't last that long, comparatively speaking. Spinning them up and down all the time is hard on a drive. So you end up having to replicate data and from time to time migrate data to new drives as old drives reach their service life. The service life of rarely used Blu-Ray media that has always been handled robotically (i.e., nothing touching its surfaces ever) is such that Blu-Ray media from ten years ago is probably still usable, the technology itself will become obsolete like DVD-RAM long before the media wears out. Not so much with hard drives, though disk arrays basically have unlimited life given typical failure patterns (i.e., if you're using RAID6, a drive develops errors, you remove the failing drive from the array, rebuild the array on a new drive, and chances of having two more drives fail during rebuild and thus losing the array are slim for a 12-drive array). So MAID has not really taken off the way we expected ten years ago.
At the time I first encountered MAID I was working for a company called DISC Storage, which had a NAS head which would automatically migrate little-used data to an optical jukebox in a way similar to what Facebook appears to be attempting. I designed and implemented the clustering function that would replicate the data between two NAS heads / optical jukeboxes, since the DVD-RAM platters were not themselves RAID'ed, as well as implemented a lot of the back end functionality for jukebox control and so forth. In any event, it looked like a NAS head but most of the files had been migrated to the DVD-RAM platters, and if you accessed one of those files, you would (at some point maybe 15 seconds later) get your data back as the file got read back onto the hard drive. It worked. But it was somewhat slow and cumbersome, because you're relying on a robot to go out and fetch the disk and put it in a drive, and disk robots then, and now, simply aren't that fast compared to media that's already in a drive ready to be spun up and read.
So anyhow, it was fairly obvious to me by mid 2003 that optical jukeboxes simply weren't going to be the future. In the ten years since DISC went under (there is a German company by that name now but it isn't the same company, it bought the name and some of the IP), I have not had any inclination to work for a company doing optical storage, because it's clear that for most problems it isn't the solution. It's too slow, too bulky, and magnetic disk drives and magnetic tape drives just continue getting bigger and cheaper every day. And now, with SSD coming on strong, optical jukeboxes look even less compelling.
So color me amazed. Optical jukebox and optical media technology essentially has barely moved on in the past ten years and what wasn't particularly compelling then, is even less compelling now. If you have need to keep data for a *long* time, this is how you do it... but frankly, I will be surprised if Facebook even exists ten years from now given the pace of innovation in the industry (though I'm just as surprised that Slashdot still exists!), so I question why they would do this rather than invest in LTO tape libraries, which have the advantage of being significantly denser.