I stumbled across Riak while searching for an error in the Node.js debugger. Apparently it uses Link Walking. This is interesting because it looks like Linked Data, but I'm not sure it is. Maybe it is. But it isn't a graph database like Neo4j. Does it have to be?
See comparisons: http://docs.basho.com/riak/latest/references/appendices/comparisons/
Interestingly, they also linked to Web Linking: http://tools.ietf.org/html/draft-nottingham-http-link-header-10