The key to Rock Band's lasting appeal hasn't been the core games. It's been the ton of downloadable content released for the titles, which just hit a significant sales milestone.
Harmonix's Chris Rigopulos says that, having recently passed a "significant milestone", over 100 million Rock Band songs have been downloaded from the game's online marketplace.
Considering there have only been three full Rock Band games (excluding the band-specific released), 100 million is an amazing number. Just goes to show, when you can hook a fan and keep feeding them content, they'll keep on paying until you run out of things to feed them.
The Past And Future Of Rock Band DLC [Game Informer]