Unlike the Miiverse communities surrounding individual games, Activity Feeds serve as a social networking vehicle on Wii U and are tied to your Wii U friends, not particular games. Your Activity Feed, for example, is only displayed to your friends and those who follow you on Wii U.
Nintendo has been steadily iterating upon the Miiverse since its introduction last year. The last significant update doubled the amount of characters each message could contain and prior to that, web profile customization was added. Nintendo has said that it expects to launch Miiverse on 3DS sometime this year.