Jelastic.com, the Java PaaS similar to Heroku, has compiled an interesting market share analysis of their more than 1,000 developers. Since their service is built on standard application servers and databases, their developers can choose which of four major open source databases they want to use: MySQL, Postgres, MariaDB and MongoDB. Granted, this is just one company's view of things, but given that the numbers are still interesting, and there are some differences between the choices by North American and European developers.
MySQL accounts for nearly half of the population to date, but MariaDB is coming on strong.
And when we look at regional differences, MySQL is stronger in Europe, but MongoDB has more concentration in North America.