There was a really interesting talk about Facebook's infrastructure at InfoQ some days ago. Jason Sobel presented the evolution of Facebook’s infrastructure over time, from the original LAMP stack to the present multi-datacenter configuration, the challenges faced and plans for the future.
Scaling the Social Graph: Infrastructure at Facebook @ InfoQ
The most interesting part of the talk is about Facebook's fbobj and assoc abstractions. Facebook places all information in Facebook objects (fbobj) that have IDs and then they interlink them using typed associations (assoc). E.g. there are associations (typed links) to friends, events, photos, etc.. That is really great when doing queries. I think HTML <a href> links should also be extended to allow for types and maybe properties. This would help building the semantic web a lot!