Facebook is battling it out with Google on many levels. One of the geekier ones is on hardware design. Both companies employ custom servers and data centers. Google takes a very secretive and closed approach, Facebook is a proponent of open hardware design.
But both companies take pride in their lean and mean data centers. One of Facebook's biggest is in Oregon and it's only a year and a half old. But Facebook is already expanding it. The most interesting planned expansion is a facility internally known as "Sub Zero" which will be an emergency-only backup facility.
As cool as it would have been, the servers won't actually be running in sub-zero temperatures. Rather, the goal is to eat up as little power as possible by completely shutting down the ones that are not in use.
The facility will act as the second backup level. Facebook has to complete backups for all of its data, all of those billions of photos and everything else. The first level backup is readily accessible and is used whenever there is a problem.
The second level backup, which is what Sub Zero will be handling, is only for true emergencies when everything else fails. As you can imagine, data here rarely needs to be accessed. Mostly, the servers will be receiving new data, storing it and then going to sleep until any of the data there is needed.
Facebook does this to push down energy consumption from the standard 4.5 kWh per rack to just 1.5 kWh for these specialty servers.
The reason why Facebook needs these servers in the first place is that it doesn't use tape backup as a last resort like many other companies do. Tape is still the cheapest storage medium and used where you need big capacity and aren't worried about latency. Google famously restored emails lost in a Gmail outage from tape backup last year.
But, while the medium may be cheap, the machines handling it aren’t and Facebook is opting for the cheaper and more "modern" solution of having actual hard drives for the cold storage. First though, Facebook has to design the hardware for the new facility, which will take several months. Once that's done, it will probably open source the plans as it has done for its other hardware designs so far.