When I think "mainstream" I'm thinking of acts that book arenas, or are the type of bands that sound like they want to. Off the top of my head - Green Day, U2, Oasis, Jay-Z, Kanye West (who's on the list, isn't he?) the most recent
American Idol winners, etc. etc.
Arcade Fire I think is probably in the same position R.E.M. found themselves in around the time of the late 80's - They were the most popular college radio act in America but it didn't quite fit to call them "mainstream". I mean, Springsteen likes them and
Neon Bible was pretty popular but they haven't quite made their
Automatic For the People yet. I'm wondering if it's even possible for a band to cross over from indie appeal to the big leagues the way they used to - The majors are pretty dumb at utilizing the internet. Otherwise, you can tell me that Animal Collective and BSS are mainstream but I really don't think they are - tastemakers just talk endlessly about them, and after awhile it seems like they're institutions when they're not. Feist has a bigger profile now than BSS proper does.
Radiohead's at #1, and they are admittedly pretty mainstream at this point. They've got a huge cult, but they're still big figures.
Even on last.fm, which necessarily only shows a tiny slice of people from around the world, Arcade Fire have 1,155,590 unique listeners.
Man, I don't know if Last.fm is the best gauge for audience. If your numbers jibe with mine (and Arcade Fire shows 900,000 or thereabouts right now. Is this real-time?) then Arcade Fire is more than half as popular as the Beatles are, and nearly twice as popular as Jay-Z. Granted Arcade Fire has wide appeal but as far as gauging the lay of the land goes, the internet tends to be really distorting. It's an echo chamber.