As you approach a million users, you are going to have to implement further along the lines of what we have discussed so far including multi-AZ deployments, elastic load balancing between tiers, auto scaling, SOA, serving content smartly (S3, CloudFront), caching DB data, and moving state information off tiers that auto scale.
As you grow your user base in the range of 5 to 10 million you will need to focus a lot more on the data tier. This would be the time to think about database federation, that is, splitting the DB into multiple DBs based on the function/purpose, sharding (splitting a dataset into multiple parts), and moving some of the functionalities to other types of specialized DBs (such ...
