At the time Facebook was building messages, Cassandra was still pretty immature/buggy (it's very solid in my experience nowadays).

Moreover, FB needed a lot of people who knew the tools. They acqui-hired a team of HBase experts. In 2010 I would have chosen HBase over Cassandra for what they were doing. In 2014, I'd choose C*. But as always, it depends on your application and what you know (or can hire for).

Cassandra has tunable consistency. A lot of people just think "eventually consistent" but it really lets you make tradeoffs yourself about performance, consistency, etc. Plus, the way they do read repair, etc even at CL==ONE makes it a lot better for messaging systems than you'd guess. I'm using it successfully with most stuff at One.