FlockDB
A distributed, fault-tolerant graph database
...It was developed at Twitter to store social graph data (followers, following, blocks, etc.) and secondary indexes. FlockDB emphasizes horizontal scalability, replication, and support for high rates of writes and updates, as well as efficient paging through very large result sets. 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. ...