Ok so this isn’t about the obvious answer that Twitter is a social network that provides status information, but instead that Twitter’s API has changed in use. Its gone from people building countless front-ends to the service (from mobile clients to Mozilla plug-ins and stand-alone applications for both Mac and PC user) to becoming a front-end in its self.
Social search engine and mobile question-and-answer service ChaCha provides its service through a Twitter page, gambling on the fact that familiarity will eventually have Twitterers visiting their site so ChaCha can vend click-through adverts. However like Facebook applications before them, Twitter pages are more likely to keep the users in the Twitterverse providing zero benefits to the content supplier.
To counter-balance this posting that rings of doom and gloom, Web Worker Daily has some services listed that have put some lateral thought into how they would incorporate Twitter into their offering, more details here.