It could be relevant if there is some fixed refresh cycle of servers. That is, newer servers can serve more end users because of the faster cpu, drives, etc. The author seems to have skipped the concept though.
Yeah, I looked through the article, but never does he mention that one rack of today's server vintage might have an extra order of magnitude more RAM, if not cores, than one from 5 years ago.
Ten years ago, four-core Barcelona Opterons were so hot and in high demand that, if you wanted to be on the latest servers, you had to show a performance boost worth the premium. Sometime this year you'll be able to buy dual processor Naples systems with 64 cores (128 threads).
I assume that their assumption is that the facebook application has grown in complexity to keep pace with better servers. There is certainly a lot more going on as they had several years ago.
