I'm going to reiterate as others have done - this is an area where a good graph database would blow all the others out of the water. I am currently using neo4j

I'm going to reiterate as others have done - this is an area where a good graph database would blow all the others out of the water. I am currently using neo4j for a web app and find it to be extremely good in terms of performance. There is really only one downside to using a graph database - they are not really scalable horizontally as you might want. They need a fair bit of resources. But in terms of querying, they would be unparalleled in this particular use-case. They are also not in infancy - they are in use in many places where you wouldn't expect them and which aren't discussed. One big area is network management - at least one major telecom uses a particular graph db to manage nodes in real-time.