Following Dries and Rob Douglas on Twitter I mentioned in the latest updates that Twitter started using Drupal for community site for the dev team. Dries has described in his blog http://buytaert.net/twitter-using-drupal that Twitter migrated to Drupal using one of their community platforms that I really enjoy. While WP has BuddyPress (and that could be ran separately as well) Drupal has few configurations for social platforms and seems like Twitter is using one of them.
There we are - https://dev.twitter.com/ . I would really like to see a feedback from the Twitter dev team for the usability of the platform. It's a well known fact that Drupal is a great platform for developers but has an non-intuitive interface for end clients which is one of the reasons WordPress is so popular right now.