Warbook was a Facebook application written in Ruby on Rails I ran by myself in late 2007 - 2008. It grew to over 16 million pageviews a day. At the time it was more pageviews than Twitter.
I scaled it using the following stack: Perlbal for load balancing, LightHTTPD for static assets, Mongrel for dynamic requests, Memcached for caching, and MySQL for relational data storage.
I used two medium instances for load balancing, one medium instance for asset hosting, 15 small instances for mongrel, one XL instance for memecached, and one XL instance for MySQL.
I used memcached as a "write-through" cache. Everything in cache was considered fresh. Every write of a cachable object would write to both MySQL and memcached. Every read of a cachable object would start with memcached first and failover to MySQL. This reduced reads on the database by 95%.
Total hosting costs were ~$2,000 a month.