Raptr shows "53,550 hrs played" and "10,378 community members"
and raptr doesn't make up the Rock Band Community Usually.. so i think t hat's fairly good.
The Cure 
My Chem. 
Played the fallout boy song and hit 0 notes (was 3rd person to do that).
In game leaderboards I am 14,204
On the facebook app 19,341
So maybe in game is just xbox and facebook is both consoles (meaning 5,137 PS3 scores)?
One of my RB3 friends apparently only played one song (probably the first one in his list alphabetically) and got around 53,000 points. He's ranked like 55,000 on the PS3 career leaderboard. That should at least be a decent rough estimate of how many copies have been sold on that platform.