Having been back in SQL land for a bit (vanilla MySQL on RDS) I have to say that I _love_ a well designed SQL database. I forgot how much I had given up in NoSQL land.

Goodreads hit scaling issues a while ago with Active Record and a single database so we broke up the data into separate MySQL servers. At that point joining data across DB servers is impossible so we went with Redshift for BI. Nowadays we would probably go with a datalake on S3.