My impressions on Wii U...

[Jumpin]

Miiverse is like Twitter - except you can friend people like Facebook as well as follow people, you can post drawings that you did usin the Gamepad, and you can post screenshots of what is appearing in the game at the time of posting.

At various points in certain games, like Mario, the game will ask what you want to say on Miiverse. If you say something, it will post a tag that get assigned to post, and that post will appear in a speech bubble with your Mii in other peoples games at the appropriate time and location.