Jason Sobel the manager of infrastructure engineering at Facebook, gave a presentation at Stanford recently. He explains how Facebook efficiently stores ~6.5 billion images, in 4 or 5 sizes each, totalling 30 billion files, and a total of 540TB and serving 475,000 images per second at peak.
You can look at the presentation here.
If Facebook’s storage is 540TB. How big is Google’s? The mind boggles.