http://drupal.org/node/861192 comment #4 - used for Drupal's test cluster

http://kb.askmonty.org/en/mariadb-case-studies/ (including Limelight networks)

Switching in early 2010 would have made you an early adopter.

My sense is that most sites use Percona Server instead of MariaDB, because Percona and xtradb (mods to innodb) have been around longer, and those changes are generally a subset of the changes MariaDB incorporates.

I use MariaDB on all of my websites these days, the largest of which tops out at around 5K queries a second. The biggest draw for me was the XtraDB engine (which is also found in Percona Server) which is a replacement for InnoDB. XtraDB performs much better on SSDs and I was able to get over 50% performance gains on some queries with the default settings. This may or may not be applicable to your specific environment, but it is a drop-in replacement so it's very easy to just install it on a test box and stress test it with your current code.

I've been using it in production as the primary datastore on a news website with fairly high traffic (10 million+ monthly uniques, 1000s of queries per second). Have not encountered any issues since switching to MariaDB over a year ago. Monty's organization places a high value on well tested and stable releases.