How Facebook, Twitter and other big sites give back to Open Source

How Facebook, Twitter and other big sites give back to Open Source: "Big sites and services like Yahoo, Facebook, Twitter and many others rely heavily on open source software to run their operations. Happily, this isn’t a one-way street. They are also giving back to the open source community, not just by contributing to existing projects, but sometimes by open sourcing their own internal projects, giving back something completely new."