Each LTO-6 tape cartridge stores 6TB of data. Each quadruple layer Blue-ray disk stores only 128GB. I am surprised that FB went with Optical Disk instead of Tape.
Optical jukeboxes/libraries for MO/DVD/CD used to be popular a decade or so ago and now considered antique. There is just not enough density with optical.
My guess:
1) Lower cost for drives (commodity drives).
2) The ability to (easily) seek to an arbitrary location on disk
3) Cost of BD burners are <$100; Cost per GB of disks is $0.025/GB. LTO-6 drives are $1000+, and cost per GB is around $0.012/GB. -- I think the cost works out actually, especially as the cost of blu-ray is still falling.
Optical jukeboxes/libraries for MO/DVD/CD used to be popular a decade or so ago and now considered antique. There is just not enough density with optical.