Here is the canonical presentation about Brad's fun with scaling Livejournal (including memcached). It's still a good read even now if you want an introduction to the whole topic:

http://www.danga.com/words/2005_oscon/oscon-2005.pdf

edit: Think about the number of decade-old pieces of software still used almost universally because they're still one of the most effective solutions in their field. That is what makes me appreciate memcached most of all.