They're not just serving up images - hundreds of thousands of images are being uploaded daily and millions more are being sent to different users every day.

It's not like a blog or digg, or even YouTube where you can just serve up static files for the most part. Social applications tend to be extremely write-heavy and every request ends up hitting the full Rails stack.

We do around 1/3 of a billion page views per month and we run on about half of those machines. http://highscalability.com/friends-sale-architecture-300-mil...