Btrfs at Facebook

It doesn't seem large to me. They have a lot of projects going on, and I don't know how much history this contains. I wouldn't be surprised if a number of assets such as images were checked in as well so that they can be built into the final site/whatever. Then I would guess there are 27 versions of jQuery and a couple of compiled binaries.

None of these things are huge but when you sum them up across a large company 800GB seems like they were really keeping themselves in check on the average.