This article takes a look at some of the methods Facebook uses to keep up with nearly a half-billion users. Here's a bit:

in some ways Facebook is still a LAMP site (kind of), but it has had to change and extend its operation to incorporate a lot of other elements and services, and modify the approach to existing ones. For example:
* Facebook still uses PHP, but it has built a compiler for it so it can be turned into native code on its web servers, thus boosting performance.
* Facebook uses Linux, but has optimized it for its own purposes (especially in terms of network throughput).
* Facebook uses MySQL, but primarily as a key-value persistent storage, moving joins and logic onto the web servers since optimizations are easier to perform there (on the “other side” of the Memcached layer).

Then there are the custom-written systems, like Haystack, a highly scalable object store used to serve Facebook's immense amount of photos, or Scribe, a logging system that can operate at the scale of Facebook (which is far from trivial).