"Consistency problems" is a red herring; Cassandra can be easily asked to sacrifice availability for consistency [on a per-request basis], if that's what you want: http://www.datastax.com/docs/1.1/dml/data_consistency

Facebook never used modern Apache Cassandra, so when they tasked a team of experienced Haddop/HDFS engineers to build Messaging, it was natural for them to choose HBase. But the things they've had to do to deal with the problems in that architecture (e.g., sharding into "cells" to deal with namenode SPOF) [1] make me think that they would have done better with Cassandra.

[1] http://www.slideshare.net/brizzzdotcom/facebook-messages-hba...