Skimlinks is a ad network that replaces links (like Amazon products) with links with the referral ID in them (or another affiliate network ID). They have been around since 2006 according to Crunchbase.
Crunchbase profile: http://www.crunchbase.com/organization/skimlinks
Company website: http://skimlinks.com/
Twitter and other social networking services have been replacing regular URLs with trackable URLs (or, more broadly and obviously, using ANY method of tracking user clicks on a site to see who is recommending what) for many years.
Here is just the first article I was able to find: http://techcrunch.com/2009/08/25/twitter-wants-to-track-your-clicks/