Tumblr (a tool where you can easily create your own blog, and share other posts from other tumblr blogs) got over 15 Billion page views a month in Feb of 2011. Those are REALLY large numbers and require some creative ways to scale the technology. Each company’s scaling requirements are unique (i.e. what worked for twitter won’t work for Tumblr) because the subtleties in usage patterns matter a lot at scale.
The good news is that these companies that do tackle these challenges share their knowledge and often open source their solutions.
Here is a great blog post on the technical tools that tumblr used to help scale their technology