I use Facebook to follow people and groups I’m interested in. The two main features of the site are the News Feed, which shows you what all the people or groups that you’re following are up to, and the Profile, where you get to express things about yourself (which, in turn, show up in your friends’ News Feeds).
Facebook differentiated itself from the other major social networking web sites (MySpace, LinkedIn, Bebo, and Friendster) by being the first to release a development platform, so that third parties could create small applications (widgets) that users could add to their profiles. This vastly increased the degree to which users could be engaged in the site.