Facebook uses TAO for graphs: https://www.facebook.com/notes/facebook-engineering/tao-the-...
MySQL is used for storing textual user content like comments, etc. See https://www.facebook.com/MySQLatFacebook
Photos are stored using specialized systems: https://www.facebook.com/note.php?note_id=76191543919 http://www.stanford.edu/class/cs240/readings/haystack.pdf
And you can see some pictures of our new data center used for cold storage of older photos: http://readwrite.com/2013/10/16/facebook-prineville-cold-sto...