Friends let friends know what they're watching
Here's a great example of social networking applied to an existing business. According to this New York Times story (registration required), online DVD-rentals pioneer Netflix is testing a new feature that lets friends recommend movies to friends, helping many of us find good movies that we otherwise have a tough time finding due to Netflix's somewhat trying interface...I had started keeping a list of movies that I read reviews about and then searched directly for those titles when I'm setting up our queue.
Essentially, Netflix is trying its own form of the social networking made popular by sites like Friendster and Tribe.net. Netflix Friends works by letting members invite other members to join their network.
The system works in two directions. Once a network of friends is created, each person in it can view the ratings and comments of others about specific films. Each member of a network can also share ratings and comments about movies with any or all of the others in the network. The more films each member sees and rates, the more value each brings to the network.