Rock Band now features over 1,000 songs; overall approximately 60 million songs have been downloaded

"The overall Rock Band music game platform now features over 1,000 songs from 390 artists available across disc-based and downloadable sources, say publisher MTV Games and developer Harmonix.
Those tracks, both free and paid, have been downloaded some 60 million times by Rock Band players, making for an impressive average per-track download count of 60,000. (That number is likely higher for download-only tracks, since songs included with base game discs need not be downloaded.)
The milestones comes just over two years after the franchise was launched with the original Rock Band for Xbox 360 and PlayStation 3. The game, and series, later came to Wii and PlayStation 2."
Source: Data from MTV Games and Harmonix, reported by Gamasutra, 25th November 2009