Earlier quoted context omitted.
I think his point is that Github is about actual work artifacts, while all other business networks are basically talk about a more or less specific topic - more flexible, but also less focused. Github is much closer to the topic at hand. There is almost no way around attaching comments to a piece of code.
But Github is about one specific kind of real work. The conversations that happen on Jive and Yammer are also real work, as are the documents made in corporate/public wikis (which are studied extensively) and the work assets made and stored in SharePoint and Google Docs. I don't disagree with the author's premise that GitHub is great and should be studied, but it only covers one small portion of one type of work.
SharePoint and Google Docs are not social networks. They are sharing solutions like Dropbox is. Your corporate wiki might qualify, but thats hard to study.