Well TBH 500 is ridiculous overkill. Even 100 would be pushing it.
And for the level of content, interactivity, media, etc etc, I can say that 500 is silly.
What sort of computing resources do you think it took for Google to develop the autopilot car? Would you have been able to determine that by looking at their homepage and the services they provide? No. That's the point. I'm not suggesting that Digg is doing anything so interesting, but most of those 500 servers are almost certainly NOT being used to support their website directly, they're being used by the business for other things.