I found it interesting that FB uses 2.5" drives in RAID 10 config. I would imagine the lower power of the drives makes up for the increased cost.<p>I would think that they're significantly slower, but it looks like Seagate makes a reasonably priced 10,000RPM SCSI 2.5" drive.
Facebook and flickr seem to be about equivalently efficient; Facebook is about 10x bigger and uses about 10x as many servers. Fotolog seems much more efficient than either though; it's about the same size as flickr, but uses only 1/3 as many servers.