From RockBandWorld, the nonexport Blitz songs range from 18,382 (Kids in the Street) to 30,200 (Pumped Up Kicks) unique Blitz players' best logged scores.
So it seems the game has sold over 30,000 units.
Now, the RBW/Facebook attach rate is a big unknown. Aswell as the count of people who are still playing today and didn't just consider it a one-weekend disposable diversion.
Rockbandscores.com is showing the high water mark of Blitz songs played in RB3 as We Are Young, with 11,082 (7982 Xbox360 and 3100 PS3).