I don't know why people are shitting themselves over a lack of four versus four player online :lol The screen can only display four people at a time without it turning into a gigantic clusterfuck. So, realistically, you'd be battling each other separately. Then when the song ends, you'd compare scores.
Guess what does that exact same thing and doesn't require all eight players to be online at the same time? The online challenges. What a concept. If you've been playing Geometry Wars 2, then you have a feeling of what it'll be like.
Then of course, I never felt compelled to once play Rock Band online, seeing as how the fun is in, you know, actually playing with friends. So there you go.