Hadn't heard of Titan before. tl;dr from: http://thinkaurelius.github.io/titan/

"Titan is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster. Titan is a transactional database that can support thousands of concurrent users executing complex graph traversals in real time."

I love titan and seeing them get acquired by Datastax makes me hope for even better support for cassandra.