This post is based on my notes taken at the talk by John Adams at LISA 2010 conference. Any mistakes, if any, are all mine. Should you be interested in other sites, check out Google, Facebook, LinkedIn.
As one of the leading social Web site with 165M users, Twitter demands a huge infrastructure support its operation. There are 700M searches and 1,000 tweets per second and can go up to almost 4,000 at peak. The number of tweets is not that impressive, but these tweets need to be distributed to numerous followers which could be several millions after one account.
These days Twitter gets 75% traffic from API and 25% from the Web. The new twitter.com Web interface heavily uses AJAX and acts as API client to its backend.
As John put it, “nothing works the first time.” His recommendation is to use the best available technology for scaling. You will need to plan and build for more than one time to get it right.