Both this piece, and most commenters, seem to miss the point about where Github's value comes from. It's not for being a social identity for programmers, or hosting open-source libraries. Those are worthy pursuits, but from a business perspective mostly serve as marketing. Github is basically Microsoft Word for programmers, and is an essential part of their workflow. Companies that need enterprise functionality aroun…
Also, keep in mind, we can't see the terms of this deal, and how much special treatment the investors will be getting from it.
The valuation is just an upper limit to the true measured value of the company until we can see the terms of the deal.