MariaDB offers performance improvements but it is a community driven project as well
The Wikimedia Foundation has announced that the English and German Wikipedia as well as Wikidata have migrated to MariaDB 5.5 as their main data base.Wikimedia has been running the Facebook fork of the older MySQL 5.1, but the improvements in the newer MariaDB 5.5/Oracle MySQL 5.5 were hard to pass.
The decision was mostly a technical one; the switch to the open source MariaDB 5.5 offers some performance improvements and makes available a few new features.
"MariaDB’s optimizer enhancements, the feature set of Percona’s XtraDB (many overlap with the Facebook patch, but I particularly like add-ons such as the ability to save the buffer pool LRU list, avoiding costly warmups on new servers), and of Oracle’s MySQL 5.5 provide compelling reasons to consider upgrading," Wikimedia's Asher Feldman explained.
But the decision was also influenced by MariaDB's pure open source roots and community.
Not all open source software is created equally, while Facebook's fork of MySQL is technically open source, in that the source code is available, Wikimedia would much rather rely on a true community-driven project, like Wikipedia itself.