We found out that Verizon -- just Verizon -- was handing over "phone records of millions of Verizon customers daily" in June 2013.
500M might be an official number that they're willing to release but (without any way to prove it, obviously) I'm certain that the REAL number is much, much higher.
For an organization who has vowed to "collect it all", it just doesn't seem likely that the NSA voluntarily went from "an estimated billions of records collected per day" to just 500M per year.
Or perhaps it's just 500M according to their own made up definition of "collect"?