Q: Why didn't you base this on MariaDB, Percona Server, Drizzle, etc....
A: We reached a consensus that MySQL-5.6 was the right choice for this, as it has the production-ready features we need to operate at scale, and the features planned for MySQL-5.7 seem like a fitting path forward for us. We will continue to revisit this decision as the ecosystem evolves.
Also from the WebScaleSQL FAQ:
Q: Why didn't you base this on MariaDB, Percona Server, Drizzle, etc.... A: We reached a consensus that MySQL-5.6 was the right choice for this, as it has the production-ready features we need to operate at scale, and the features planned for MySQL-5.7 seem like a fitting path forward for us. We will continue to revisit this decision as the ecosystem evolves.