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.
You missed the tiny little word "artifact". The discussions on Yammer and Jive are work related and are part of the process, but are not rarely directly attached to an artifact. You would talk about the artifact, but its not "there". It is produced somewhere else, in the depths of IT. That doesn't mean that Yammer ain't interesting.
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.
I did not miss the word, just disagreed with his premise that assets are an important distinction.
That being said, wikis are not hard to study, its incredibly common and they are high on the list of the favorite things to discuss among collaboration researchers and have been for years - I've experienced this working with several, but you can also look at research such as http://www.citeulike.org/group/1136/article/114322 to see more.
Github is much closer to the topic at hand. There is almost no way around attaching comments to a piece of code.