Facebook has released one of its core piece of infrastructure called Tornado under an open source license.
From the Facebook announcement:
Tornado is a relatively simple, non-blocking Web server framework written in Python, designed to handle thousands of simultaneous connections, making it ideal for real-time Web services.
Tornado was originally developed by Friendfeed to power its real time web infrastructure prior to it being acquired by Facebook.
Check out the other open source projects from Facebook.
Read in detail about the technology behind Friendfeed's web server Tornado here.