Apple acquired FoundationDB but from what I have heard it’s not really used much. The FoundationDB founders have left Apple and are working on other things. Cassandra is the main datastore for iCloud data.

Are you sure? The latest SIGMOD paper on FoundationDB says it's in active use. My impression is that iCloud is built on both Cassandra and FDB, and that FDB (via Record Layer in particular) is used extensively.

Well, since we're exchanging unsubstantiated rumors: from what I have heard it's exactly the opposite. FoundationDB is being used for transactional data and works very well. Its active development by Apple would seem to confirm that.

These are different technologies with different use cases. FDB typically is used when there are strict ACID requirements.

Apple uses both

I am really curious. It would make sense to use Cassandra as low logic, high volume simple storage. And then use something more relational / graph and ACID on top of it