"Almost all our servers are running open-source software. Our Web servers use Linux and Apache and PHP. Our database servers run MySQL. We use memcached to help keep the site snappy. Some of our behind-the-scenes software is written in Python and Perl and Java, and we use gcc and Boost for the parts that aren't. Our developers use Subversion and git to keep track of their work. The list goes onlike many Web sites, we use it from top to bottom."
Looking at this you can see MySQL and PHP so I'm guessing at the highest level, a LAMP architecture. Don't know how they go about web farming, scaling though.
http://blog.facebook.com/blog.php?post=2223862130
From the blog post:
"Almost all our servers are running open-source software. Our Web servers use Linux and Apache and PHP. Our database servers run MySQL. We use memcached to help keep the site snappy. Some of our behind-the-scenes software is written in Python and Perl and Java, and we use gcc and Boost for the parts that aren't. Our developers use Subversion and git to keep track of their work. The list goes onlike many Web sites, we use it from top to bottom."