Miku is a Vocaloid, and Vocaloid is a voice synthesis software program developed by Yamaha. From what I've read, Vocaloid is not an anime nor manga; it's not a game like Kancolle; it's not a visual novel like Fate/SN; and Miku is not a mascot for a gaming company like Super Sonico or a Windows mascot like OS-tan or an IE mascot like Inori Aizawa. There is a video game series, but originally, she is a singing voice synthesizer software.
Miku herself is a cute humanoid persona for Vocaloid (picture from Vocaloid website)
But the Vocaloid interface itself looks like this (picture from Wikipedia)
It doesn't look like a software that just anyone would use.
Aside from her really nice character design, what made Miku become so popular? She is all over the internet and has a lot of fanarts, figures, and cosplayer pictures everywhere.