An interesting, if perhaps flawed, thesis, and a great meme: 'dark social'. The general idea is that, if someone visits somewhere with a long URL then the chances are that they clicked it, rather than typed it, so, if the referer was not via a website, then the author thinks that most of it must be coming from email or IM clients. So it's social sharing, but not being captured by the statistics.

Well, maybe.

It is possibly flawed because email and IM are not the only ones that don't reveal a referer. Many browsers deliberately avoid sending a referer, so do some web crawlers and so do browsers when a bookmark is clicked or sometiems when a browser is opened with previous links open in tabs. Not to mention a host of mobile apps that do not identify themselves but that aggregate RSS feeds etc, and some URL shortening services that blank the referer. So, while it's a plausible hypothesis that email and IM might account for many clicks, I don't think this method gives a clear indicator of how much it happens. It is especially uncertain as a fair number of social sites send email notifications with links in them so the 'dark' stuff may well originate on the more visible social web. What might be more interesting would be to discover how many webmail client referers are visible - if you could identify rough proportions for webmail vs mail client users, then it might at least hint at the relative percentage of email clickthroughs.