I wonder if this is actually cost effective.
In my data hoarding days (as a consumer, not FB scale), I found burning media an archive tool to be much more costly (and less reliable and more physical space) than simply using multiple hard drives.
Now, it could very well be that FB is able to buy burnable media at a much cheaper rate than consumers (i.e. perhaps there is more margin in media that massively bulk purchasing can reduce and the type of HDs that FB would buy would be more expensive than consumer drives, it could also be that actively used burnable media would be more reliable than actively used hard drives. while it's cold storage, its not frozen storage that is rarely used, with that said, the jukeboxes are probably expensive and suffer more reliablity issues than the hard drives), but on the consumer level, it just didn't seem to be a doable thing.
Ex: 8TB HD could cost $120. That's ~ 400 25GB single layer blurays. Now, 400 disks of optical media no matter how efficient one can store them will take up a lot more room than a single 8TB HD. For the 400 bluray blanks to be cheaper than the HD, they would have to be less than 30cents a disk. Glancing at amazon today, the cheapest I see for 25GB BDR blanks (and dual/triple/quad layer blanks are more expensive per GB, i.e. a 50GB blank would be $1.6 a disc) is about 40cents a disk. At that price level, you are better off just buying/copying to multiple disks for even better disaster recovery and it wont really cost you more than to store everything once on optical media.
anyways, if anyone could point out flaws in my assumptions (or why at FB scale the answers are different) I'd be interested.