Well, there is a vast difference between "every fortune 500 company is using Github in some capacity" and "every fortune 500 company is paying for the Enterprise version of Github." I think it'd be an easy sell on a lot of companies, since, as you say, many of the developers are already using it. Heck, I even want to convince my company to get the Enterprise version of GitHub, but we're a non-profit and too small for it.