What development tools do Facebook, Twitter and Google use to develop their websites?

Facebook has been used PHP since he came into the virtual world. Now they have their fully customized PHP framework.

Google Search - Python (http://python.about.com/b/2006/11/17/creative-search-technology-with-python.htm)

Google Maps Frontend - JavaScript (Which language is used in google maps frontend?)

GMail Frontend - JavaScript (http://en.wikipedia.org/wiki/Gmail#Code_changes)

 Python has been an important part of Google since the beginning, and remains so as the system grows and evolves. Today dozens of Google engineers use Python, and we're looking for more people with skills in this language." said Peter Norvig, director of search quality at Google, Inc. 

Twitter:

Ruby on Rails for the frontend and scala for some of the backend: http://www.artima.com/scalazine/articles/twitter_on_scala.html