Even at 90 million users, with anything approaching a reasonable level of activity, we're not talking about serious data. 90 million rows of denormalized data

Even at 90 million users, with anything approaching a reasonable level of activity, we're not talking about serious data. 90 million rows of denormalized data isn't a big deal, and if I had to guess, their ops per second is probably no higher than what a dedicated single, or maybe a small master-slave postgres deployment could handle. Again, something a DBA would yawn at. And I say this as someone who scaled up an API for a service that plugged into multiple ad networks concurrently for a total of billions of impressions per month with a high level of reliability. Using NoSQL and an RDBMS combined. People who want to preach the NoSQL message should probably have some actual experience. Otherwise, it just makes very viable NoSQL solutions look really bad.