I played around with several. But project never got off the ground due to layoffs that killed projects.
I know lots of people who have implemented graph stores with great success. One example:
http://www.bbc.co.uk/blogs/bbcinternet/2012/04/sports_dynami...
Another is a multibillion dollar retailer (not sure if it's public so I'll leave the name out) uses stardog to good effect. LOTS more out there.
>>We paired on setting up Neo4j, the only open-source graph database we could find with a working Python API, but found it fiddly and hard to get help.<<
The Graph Stores do seem to play better with Java. Neo's getting a lot of ink these days but they are far from the only game in town.
>>Facebook themselves use MySQL[1], so between that and my own first/second-hand experience, I'd call it far from obvious that a graph database is the most appropriate way to store social information.<<
They aren't using MySQL the traditional way. They undoubtedly would have made different choices had they started when Disapora did. And they also use TAO, a homegrown graph store of sorts, FYI:
http://dl.acm.org/citation.cfm?id=2213957
It is sitting on top of MySQL at some level, as this is where objects are stored as "source of ultimate truth".
IIR the quote correctly when I invited couple FB DBAs (pre-Mark Callaghan) to speak at a meetup, "I don't think there's a single join in the facebook codebase". That might have been a slight exaggeration, but MySQL at Facebook is not because their recent needs are for a relational db.