The Architecture Twitter Uses to Deal with 150M Active Users, 300K QPS, a 22 MB/S Firehose, and Send Tweets in Under 5 Seconds
It happened gradually so you may have missed it, but Twitter has grown up. It started as a struggling three-tierish Ruby on Rails website to become a beautifully service driven core that we actually go to now to see if other services are down. Quite a change.
Source: highscalability.com