Services like AppAnnie (site) can give public estimates of downloads and revenue, but generally speaking those numbers are pulled across iOS, Android, Facebook, Windows phones, etc, etc, etc. Especially when that combined number bumps them over a milestone number like a million.
Then there's nothing to stop them just plain lying.
Need professional proofreading, editing or writing that won't break your budget?
Contact me at youarelowonhealth.com or via PM.