Use cases will vary, but where I work we've scaled a simple MySQL master/slave setup + memcached for some caching up to around 5 million active users per day.

Use cases will vary, but where I work we've scaled a simple MySQL master/slave setup + memcached for some caching up to around 5 million active users per day. The largest table we regularly touched (no caching) had just over 1 billion rows. Most projects will never reach this scale.