From Social Patterns
Public website like blogs and microblogs, like twitter, and sites like flickr where the default for photo posts is public, allow their authors to broadcast to the world. Include interactions in the interface that allow users to follow or subscribe to the author or the author’s content. These one way relationships are often just fans of the author or their content. Fans have an implied difference from friend, in that the relationship is usually one-way and there is often little or no direct interaction between the fan and the object of fandom other than consumed broadcasts.
Mybloglog allows users to add a widget to their blog and track who visits. The widget declares the implicit community around the site, whether the site owner has any interaction with them or not.
Twitter’s Followers and Following grid and full lists are, in essence, a list of fans for a person or their tweets. To some, collecting followers, has become a game or popularity contest.
Bands have long-used mySpace to build up their fan base. They offer music, information about gigs and new releases and allow their fans to interact with them and declare to the world their fanhood. Although the service calls all connections "Friends", these are clearly Fans.