Miiverse
From Inkipedia, the Splatoon wiki
Miiverse is a social network allowing conversation about video games for Nintendo's gaming system.
Messages sent to Miiverse thorugh the mailbox in the plaza may appear in graffiti and on billboards for other players in Splatoon. Other players in Booyah Base will display their most recent mailbox message when approached, or zoomed in on. During Splatfest, the amount of Miiverse messages shown increases.
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.