Re: 1) Looking at the prices on Newegg, 1.5TB(native) LTO5 tapes run about $30.00, so $20 per TB. Write-once BluRay looks to be about $32/TB, and rewriteable is $80/TB.

Re: 2) Although it would take an hour or so to search through an entire tape if all the files were written in a single session, normally backup software writes files in sessions containing a few GB. You can seek to a specific session in about 30 seconds average. So if you put about 4GB per session, it would take a max of approx. 30 seconds to read in that session to retrieve a file. So figure about a minute to retrieve any file. Also, LTO5 transfers about 140MB/sec, vs. 6x BDR disks (what I'm seeing for cheap right now) is only 27MB/sec.

3) IIRC most tape storage technolgies are closed source. FB has been heavily pushing their open compute project. Using BDs get them past a lot of licensing problems.

LTO is an open standard: http://en.wikipedia.org/wiki/Linear_Tape_Open

I don't think LTO is any more proprietary than Blu-ray.

and LTO has way less licensing problems than Blu-ray and LTO-6 has a max of 2.5TB. LTO is also guaranteed to to read tapes from 2 generations earlier and write one generation earlier. Blu-ray has no such long term guarantee.