> They don't scale to Google or Facebook operational sizes.
You may or may not know this, but the primary datastore used at both Google and FB is MySQL. Sure, they use replication and sharding, but I would strongly argue that MySQL with sharding scales better than some multi master NoSQL thing like Cassandra.
Related, you should check out https://github.com/vitessio/vitess if you haven't seen it. It's what Youtube and others use for their primary data store in production.