Social games, which rely on fellow players to advance a player's progress, are also driven by storylines (albeit, weak ones). But those storylines, like soap operas, seem to find new extensions, designed to keep players playing ad infinitum. As we've seen with several TV shows, sometimes it's better to announce a show's final season in order to boost ratings... the proverbial "go out on a high note" exit.
With games like Mobsters 2 and Mafia Wars, players do tasks to push the story forward. True, most don't care, but for those who do keep track of power players, their actions and what needs to be done before advancing, the story is the hook that keeps them playing. While none of the big games have been given end dates (yet), would you prefer the game to just stop working or would you want the story to end?