Graphs are being widely used in multiple domains, including, but not limited to, medical research, transportation, auditing, and financial risk analysis. For finding out more about modeling, the Neo4j mailing list on Google Groups is a good place. Occasionally, there are interesting modeling problems that people get down to solving. Alternately, a showcase of which problems people are solving using graphs in general and Neo4j in particular can be found at http://gist.neo4j.org/. For more use cases where Neo4j has been used, http://neo4j.com/use-cases/ is a good resource. Following @neo4j on Twitter is a good way to keep up with the happenings in the Neo4j world.