On good hardware there were PostgreSQL instances running a billion queries a day back in the 9.0 days. Often these are actual accounting apps where reporting matters and so NoSQL would be a very poor fit. For example the French government uses it to distribute welfare program dispersements. The Wisconsin Courts also uses PostgreSQL at loads in the billion per day range. I know there are larger instances out there.
Now a days we are talking about thousands of concurrent users and up to 350k reads/sec on high end machines.
