Why do you find it interesting that they didn't switch? What is the compelling reason to switch if you don't care about Oracle owning MySQL?

The only difference I remember from having been forced to use MariaDB in the past was that one of the 5.x releases didn't support NOT NULL virtual columns when the equivalent 5.x MySQL release did support them... to me, MariaDB is just a strange, slightly incompatible fork, but maybe someone else here has more experience with both. (I prefer postgres)

Their site(use case) looks almost similar to stackoverflow.com. The stackoverflow.com has few MS SQL servers with a Memcached as cache. I guess if reads are the heavy load it makes perfect sense just to continue with your current SQL DB + sharding.