[scylla]

Yup Facebook and Yahoo are PHP. Google/Amazon/eBay/Walmart.com are all primarily a combination of Java and C++.

[nbm]

Facebook uses a combination of primarily PHP(1) (mostly front-end, rendering, some async jobs and scripts) and C++ (mostly back-end services, other bits of high-performance infrastructure, HipHop for PHP), with some Java (especially in the Hadoop, Hive, and similar areas), Python (mostly automation, scripts, infrastructure), and so forth.

From what I understand, Yahoo! has a similar mix of technologies.

In both cases, you really can't say "are PHP".

(1) Modulo some in-house and open source extensions (like XHP) running on HipHop for PHP.

[Xcelerate]

Although Facebook uses Erlang for chat.

[simonb]

... and Haskell for programmatically manipulating said PHP code base. [http://www.haskell.org/haskellwiki/Haskell_in_industry]