At Facebook, we are always considering alternative database engines, whether they be in the MySQL family or outside of it (ie. PostgreSQL, etc...).

We are very committed to open source, and as such we watch Oracle's work on MySQL very closely. The technical work they have done on releases since the acquisition is outstanding, but they do tend to have community issues which can be concerning.

Currently we are running MySQL 5.6, which has a lot of interesting features that MariaDB does not have yet since it forked from MySQL 5.5.

MariaDB 10.0 beta has many of the same features, but was only just released as beta and will need time to bake before it could be used in production. Once it has progressed to a more stable state, we will definitely evaluate it further.