Alex Payne, API Lead at Twitter gave a fascinating presentation at Web 2.0 Expo San Francisco, 01 April 2009 [1] about why Twitter uses Scala. He explains why they like the language, how they have gained performance with it and the difficulties they encountered on the way. You may like to take a look at his presentation slides [2] or read reports on the talk by: Kate Greene from Technology Review [3], Nicole Ferraro from Internet Evolution [4], or Cade Metz from The Register [5].
Links:
[1] http://www.web2expo.com/webexsf2009/public/schedule/speaker/3485
[2] http://www.slideshare.net/al3x/why-scala-for-web-20
[3] http://www.technologyreview.com/blog/editors/23282/?nlid=1908
[4] http://www.internetevolution.com/author.asp?section_id=466&doc_id=174573&f_src=internetevolution_sitedefault
[5] http://www.theregister.co.uk/2009/04/01/twitter_on_scala/