Advertisements. It doesn't matter if individuals look at them or not, it has a built in feature that allows its advertisers to target individuals based on their profile information. For example, if your single you will more than likely see online dating or single people advertisements on your Facebook page. Unlike web sites that use affiliate marketing, Facebook receives money for every number of times the ad appears on a page.
Other web sites that provide information usually use affiliate marketing... For more information on the topic you may want to visit http://doctormoneymaker.com which explains affiliate marketing in depth. Hope that helps.