Programs which are built to run on an existing platform and utilize the attributes and data available to that platform. Applications can develop their own userbase which can often number in the millions and are a major reason why users visit social networking sites. Application can also be custom-made and branded to appeal to the audience of a specific advertiser.
For data on Facebook applications, see: www.appdata.com.
For custom applicaton creation, see: BuddyMedia, Context Optional.
For advertising within applications, see: AdNectar, Appssavvy