Facebook stores billions of cat pictures and other images users share for perpetuity in a drastically different way than even a year or two ago – and it's all in the name of reducing power consumption.
One of the biggest energy costs at many data centers is cooling. Giant HVAC cooling systems, usually using chilled water, blast cold air in ducts under the raised floors of data centers and direct it up through the racks of hardware. These systems then exhaust the heated air that comes out of the "hot lane" behind the racks to the outside. All those pumps and fans and heat exchangers consume a lot of electricity.
Facebook’s Cold Storage facility in Prineville, Ore., however, uses “free air cooling,” or the process of harnessing the evaporation of water into the air to remove heat.
More details here.