We've had some early successes with DataStax Enterprise Graph (DSE Graph) for handling large graph databases. Built on top of Cassandra, DSE Graph targets the type of large data sets where our longtime favorite Neo4j begins to show some limitations. This scale has its trade-offs; for example, you lose the ACID transactions and run-time schema-free nature of Neo4j, but access to the underlying Cassandra tables, the integration of Spark for analytical workloads, and the powerful TinkerPop/Gremlin query language make this an option worth considering.