It is not all take, take, take for twitter.  They have posted a new Twitter Open Source Directory where they list all of the open source projects that the company is involved or has contributed to.  It is not clear if they are involved on an ongoing basis with each project or not.  

The description on the directory page states simply:

"Twitter loves open source.
Twitter is built on open-source software—here are the projects we have released or contribute to."

The types of projects they are involved in are as follows:

Ruby - 10 projects
Scala - 7 projects
Java - 6 projects
C / C++ - 3 projects
Tools - 3 projects

It is not surprising that the open source project contributed to most are Ruby projects as twitter is built partially with Ruby / Ruby on Rails. 

We applaud twitter Inc. on their openness to share this information as it will be a huge help to developers and companies to gain insight on twitter's infrastucture and the open source projects and tools they use to achieve massive growth and scale. 

Kudos also to twitter for the time and money invested to help build out these open source projects, no doubt their contributions to the community will help thousands of engineers and many companies.