FlockDB
A distributed, fault-tolerant graph database
...It is not a general graph database in the sense of supporting complex multi-hop traversal queries or sophisticated graph algorithms; instead it focuses on the core problem of storing and querying directed edges with attributes such as sort order, state (normal, archived, removed), and position. Edges are stored both in forward and backward directions to facilitate queries in both directions. The project is now archived and in read-only mode, meaning it's no longer actively maintained by Twitter.