Scalling an application is easy. You just stick a load balancer in front of more and more webheads. The hard part is scaling a database! Twitter is still run

Scalling an application is easy. You just stick a load balancer in front of more and more webheads. The hard part is scaling a database! Twitter is still running a single master database system and - from what they've put out - only three database servers. They might also have some pretty, but wasteful queries that do a lot of joins. Joins are not good for high-hit applications.