at the time they picked HBase because it was easier to repair missing replicas. Hdfs namenode juat schedules block replication. Cassandra (at least the version they were considering) had to work a lot harder at replacing a missing replica. Remember they had a version that didn’t have workable merkel trees.

I also heard they had an issue with network flaps causing unrepairable inconsistency. I never got the full scoop on this.

I had worked with the engineers directly responsible for the HBase choice back when. All water under the bridge now it seems.

My guess is operational experience, though the article mentions it was necessary to take advantage of Lightning. I can't think of another business on the planet that has pushed MySQL (and RDBMS' in general) further than FB.