This recent June 2013 post goes into some detail into explaining the transition from relationship databases to objects with associations for some data types.
https://www.facebook.com/notes/facebook-engineering/tao-the-power-of-the-graph/10151525983993920
There's a longer paper available at https://www.usenix.org/conference/atc13/tao-facebook’s-distributed-data-store-social-graph