http://anand.typepad.com/datawocky/2008/06/indias-sms-gupshup-has-3x-the-usage-of-twitter-and-no-downtime.html
* Users: Twitter (1+ million), SMS GupShup (7 million)
* Messages per day: Twitter (3 million); SMS GupShup (10+ million)
runs linux using a three-tier architecture with mysql and JBoss app servers.
Jeff.