LinkedIn is one of the most successful freemium networks at present. Although not strictly the most economically successful, LinkedIn's large user base is particular well targeted and active. The 'freemium' business model refers to a site that offers their core services to the public for free but who makes revenue from the small percentage of the user base that pays to have access to premium services.
LinkedIn has managed to grow steadily over the last eleven years by slowly introducing new products and services to its users. It has a multi-faceted business model that doesn't fall into any one traditional internet commerce model but rather, is made up of aspects of various models most prominently the advertising and subscription models. Part of the success of LinkedIn lies in this multi-layered structure. Not all aspects of the business build revenues in the same way, but as a combination they have placed LinkedIn in a social network category of their own. Although they don't have the numbers of other social media giants Facebook and Twitter, LinkedIn's approach to making revenue is also significantly different... but more about that later.
This is an info-graph that gives a brief outline of some of the major financial milestones of the company thus far:
See published info graph here
More user information and statistics can be found here