Miiverse was a social network that launched with the Wii U platform. A part of the Nintendo Network, it allowed for users to connect with friends and with strangers with similar tastes in games. Some games also contain features that connect the game to Miiverse. One such example scenario is for allowing the display of messages left by others that have died at the same obstacle in the game as the player.
Since the launch of Miiverse on the Wii U, Nintendo expanded the social network to allow users to browse and post to Miiverse communities from web browsers. A 3DS firmware update in December of 2013 enabled the use of Miiverse on the handheld. Nintendo Switch, the immediate successor to the Wii U, does not currently support any sort of Miiverse functionality.
The Miiverse service was discontinued by Nintendo on November 7, 2017, disabling all Miiverse-specific features in Wii U and 3DS games. An official statement by Nintendo on their North American support website claims the decision was made "to end the service at this time because, among other reasons, many users are shifting to social networking services".