Last night, David Letterman's musical guest was an enormously popular Japanese pop star who has opened for Lady Gaga and racked up an estimated 88 million views on YouTube. She performed her song "Sharing the World." She is also not human.
She's also a vocal synthesizer software with no corporeal form, the VR personification of a software called Vocaloid developed by Crypton Future Media. She's not the only Vocaloid singer, but she's the most popular.
Miku has an ardent fan base, and it's not unprecedented for Letterman to give East Asian pop stars a slot (he had Korean girl group juggernaut Girls' Generation on in 2012). The only unusual thing about Miku is that she's a software program and not, you know, a biological being.
Hologram music performances also aren't unprecedented in the U.S. The band Gorillaz uses holograms to project cartoons while they perform, and concert festivals and music awards shows have created holograms of beloved deceased performers like Tupac and Michael Jackson. (Miku is projected on stage using the same "Pepper's Ghost" optical illusion used to make a representation of Tupac.)