From the FAQ on that page: "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 ch

From the FAQ on that page: "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." MariaDB 10 just hit after the launch, much less conception, of Webscale, and still doesn't have all of the features from MySQL 5.6 in it.