The actual businesses generally don't. It's funny how RDBMS fans always say "businesses x, y and z were built on MySQL" and forget to mention that they were actually using MyISAM-based versions of MySQL that had no ACID and barely any referential integrity constraints. Most of the "mongodb is web scale" crowd is coming from the same ideology as the people who were hating on MySQL 10 years ago.
You should probably tell the millions of companies actually using them for things that they're wrong. I'm sure they'll feel very silly.