Re: Facebook, as jbellis mentions in this comment thread, Facebook shards HBase, so it's not able to support what they need without some major additional complexity.
Re: eventual consistency. It's tunable to the need of the user. It's a trade-off. Tunable consistency becomes nice when you want a system to be always available. See the Amazon Dynamo paper, on which Cassandra is partially based.
I think many underestimate how many are using Cassandra in production. Spotify, eBay, Walmart, Netflix, Ooyala, NASA and others - one list is found here: http://www.datastax.com/cassandrausers.
