Facebook is huge. It serves hundred of billions (literally) of pages a month, and 1.2 million photos every second. Wow. It's generally considered the world's largest web site. I'm waiting for an episode of Build it Bigger to air talking about Facebook, but I doubt they'll do it. Software scaling is hard to show in TV (but data center pictures are exciting to some, I suppose).

Operating software, databases and infrastructure at Facebook scale is a massive and complicated undertaking, and they actually do a lot of it on open-source software.

Pingdom takes a look at how Facebook does it, and describes some of the open-source technology the company leverages, in an interesting article called The Software Behind Facebook. It's worth reading.