Hope replying to old posts isn't frowned upon but I wanted to share something I made which relies heavily on Kythe.
It's a open-source web application which uses Kythe to extract function references between projects on GitHub.
That shows all the projects which I've indexed that use gson and if you click the methods it'll actually show you the code which used gson.
Moving forward I hope to start showing trends. For example, if google/gson2 came out and people stopped using google/gson; I think it would be cool to see that trend happen in real-time.