Miiverse

Miiverse is a social network incorporated into Splatoon, allowing conversation about video games for Nintendo's gaming systems.

Description
The mailbox in Inkopolis Plaza allows sending messages to Miiverse. Drawn messages, known as tags, may appear as graffiti or on billboards for other players in the plaza as well as in stages. Tags, as well as text messages, appear over other Inklings in the plaza when approached, or zoomed in on using. The tags can be disabled via the Options screen.

Splatfest
During Splatfest, the amount of Miiverse messages shown increases. Drawings can also be seen in firework explosions, and on neon signs scattered throughout the sky.

Trivia

 * Miiverse for Splatoon is split into three regions: North America, Europe and Oceania, and Japan. Players from one region will not see Miiverse Mailbox messages from another region.
 * Some, if not all, graffiti in Inkopolis Plaza is implied to be made by jellyfish; as stated in a developers' interview, jellyfish are known to create a large amount of graffiti.

Miiverse links

 * North America
 * Europe/Oceania
 * Japan