Why MySQL and Cassandra were not selected for FB messaging

The part about Cassandra and replication is wrong. One can configure replication factor to be 3 and still make writes with consistency level one - which means the other 2 writes will happen async and won't block the client.

The part about write ahead logger is also useless and cassandra also does that. it's called commit log in cassandra.

It gave me the impression that the author really doesn't understand cassandra.

As for the decision of facebook to not use cassandra - i guess it has more to do with maintaining data hotspots or Map reduce like computation on the data than replication and scaling factors. Even though cassandra has key cache and map reduce from version 0.6 they are still not very mature.