A colleague pointed me at this great article which talks about how the architecture of MySpace evolved as the site became more successful.

The site started out initially as a Perl/Apache/MySQL prototype, moved to SQL Server for live (2 Web servers and 1 DB server - which handled up to 500,000 accounts) and ended up with many 64 bit SQL Servers and at least 150 ASP.NET servers! A very interesting read.