The valuation imho is a bet that GitHub will be ingrained in developer's workflow and continue to occupy developer mindshare for at least the next decade. Some reasons have already been mentioned in the comments here: * Revenues - It's already the dominant player in git hosting and has better enterprise plans, * Mindshare - GitHub is not just git hosting, it's wikis, issues, gh-pages, atom, and more to come. Each of them are good products in their own right and also create a lot of value by being under the same roof. * Platform - Github is no longer a product, it's an emerging platform. There are plugins / extensions that augment the power of GitHub; zenhub.io (as someone mentioned here) - a case in point.
And one point that every comment here has missed is Github's value as a latent hiring marketplace. Unlike LinkedIn / Resumes / Referrals, a github profile is a much better signal for an employer. Punchcards, followers, projects built and contributed to build a very comprehensive visual of a developer, his coding style, tools used. As Github gets more popular, more employers will find value in this providing a reinforcing loop getting more developers to be creating profiles and maintaining them.