It appears that Apple may be planning to discontinue the Cards app that it first released for iOS in 2011. Opening the app with devices running both iOS 6 and iOS 7 results in the following message: "The Cards service is no longer available."
The Apple.com page for the app has also been removed, now redirecting to iPhoto '11 for the Mac.
While it is unclear whether or not the app will be revived with an iOS 7 update, the wording indicates that it will be quietly pulled from the App Store. Currently, while new cards cannot be created or sent, old cards can be viewed.
Cards, which is free, is still available for download in the App Store for the time being. [Direct Link]
Thanks, Jesper!
Update: An Apple support document confirms that the Cards app will be discontinued on September 10, 2013.
Article Link: Apple's 'Cards' App Appears to Be Discontinued [Update: Confirmed By Apple]