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.