Open source graph database

I agree with @Reinderien, you should bench these products. But I would also add OrientDB in the mix. Quoting its author in this comment:

I'm working to OrientDB. It's a NoSQL hybrid document-graph dbms with special operators for graph operations. The main difference is that you can query it using SQL language with some extension like:

select from People where friends TRAVERSE(1,7) (name = 'Ayende' and surname = 'Rahien')

This means get all the people that have any relationship of friends with you up to the 7th level of separation.

According to its website, the performances looks pretty decent. Worth the look at least.

I think you can pay attention at Flockdb too.