Facebook is a maze for developing data store and access algorithms. Their scale allows and actually requires to find out new ways of storing information.
Facebook uses 3 levels of persistent storage: hot data is stored in Haystack storage – it’s lowest level is the prototype I used for Eblob. I would describe Elliptics storage with DHT, i.e. multiple servers to store one data replica, as this level of abstraction.
Second comes so called WARM data – data which is accessed by 2 orders of magnitude less frequently than that at HOT level. Warm data is stored in F4 storage, Facebook describes this split and motivation in this article.
I saw Elliptics installation with one disk per group, i.e. without DHT, as a storage for this access level of the data.
And the last is a COLD storage, something weird and scientifically crazy like robotic hand with a stack of Blu-Ray disks.