Hacker News Reader

eBay: Perl

Amazon: Perl

Twitter: Ruby on Rails, Java

Optimizely: Python

LinkedIn: Java

Craigstlist: Perl

flickr: PHP, Perl, Java

Tumblr: PHP

Twitter moved off to Java last I heard if I read this correctly. Not sure if they still use any rails.

Most of their job postings still mention Ruby, so I suspect a big portion is Ruby/Rails based.

LinkedIn: Java/Ruby on Rails