Many large websites use multiple programming language in their back-end depending upon the particular feature you're utilizing. The back-end of Google search is powered by Python, although they also make use of PHP, Java, and other language at various locations. (The creator of Python is employed at Google, by-the-by.)
Amazon and eBay primarily rely on CGI, which isn't a programming language, but rather an interface that allows a web application to communicate with a program on the server, such as one written in C. These (typically and predominantly) offline languages that are compiled tend to offer greatly improved processing speeds vs. interpreted languages, such as Python, PHP, or Ruby.
Microsoft-owned websites tend to rely on ASP.net, but although many would say they strictly rely on this technology, it's not true. You'll find Python code and others within Microsoft projects, as well.
Wikipedia relies predominantly on PHP, and Twitter relies mostly on Ruby on Rails, although their most intensive features are handled by Scala, due to scaling problems they've had with Ruby.