Web apps scalability

Along with web 2.0, web traffic is getting increased day by day. Individual users as well as enterprises are getting into the the web eyeing on the SAAS type of application and moving away form the desktop apps.
The amount of traffic generated by these apps are tremendous, Just to name few

• Over 100 million video downloads per day from YouTube.
• More than 4 billion queries per day on FlickR.
• Do I need to mention Google, .........

How these apps handle the traffic, whats the architecture behind, what are the technologies empower them ..... are some of the questions come to our mind,

I came across a good web site with some of the very good informations about these apps, answering above questions. It talks about how much scalable they are , and how it has been achieved and the technologies are being used.

Google Architecture info - here
YouTube Architecture - here
FlickR Architecture - here