I don't use github so much, i prefer gitlab. It's open source, free*, and gives you options for private repos. git.machina.hr is an example of gitlab server. Pretty much the same experience as github.
I plan to create a new gitlab instance with signup enabled, so anybody willing to contrib will be able to do it. Anybody will be able to create his own private/public repos for his/hers plugins.
Dunno, github is cool and all, but i think that there is a need for a public repo with specific stuff for C2. It will enable much more options to develop and scale. Maybe i'm wrong? What's your opinion?
* free as in free code but not free hosting/server