Facebook for a long time didn't use the vastly (and I mean vastly) improved open source version of Cassandra, instead opting for their internal fork. Instead of choosing to do so, I believe they have now switched to HBase, mainly for its easier consistency model. So I would take their advice with a grain of salt, because it's probably based on their experiences with an old fork.
There are a few people (YC companies even, alas) who are very vocally negative about Cassandra, but I also saw some of those same people ignoring direct advice given to them in #cassandra on IRC, and then turning around and bashing it when it didn't work as planned. Simply following the advice could have made for a completely different story.
I suppose the lesson to learn is that you need to develop software in a way that simply won't allow developers to shoot themselves in the foot, because people never want to blame themselves for doing it, they blame the gun.
That's exactly what I'm talking about: that facility9 blog post explaining why they chose HBase had many factual errors about Cassandra when it was posted, and had to be revised after several respected people in the space contacted the author.