Facebook is the largest site in the world. Some of the statistics are staggering. Please take a look at this article on Facebook use and customization of open source to scale.
- 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).
If you have a bit more time to explore, you may also want to take a look at this presentation filmed during QCon SF 2008.
Aditya Agarwal discusses Facebook’s architecture, more exactly the software stack used, presenting the advantages and disadvantages of its major components: LAMP (PHP, MySQL), Memcache, Thrift, Scribe.