usually, BMI and such just take a random sampling of radio airplay (3-4 months)
to generalize what the airplay is so basically hit rotation artists 
get the money.
stuff that gets played ONCE usually falls through the cracks or you 
get a bit of
the unnaccounted for funds at the end of a reporting period that gets 
distributed out
to everybody at the bottom of the trough... or not....

however, i do believe most USA collecting agencies are switching over 
to get info
from the radio data services that actually do track every song (the 
systems are already in place)

somebody here may be more up to date on this -


in canada, the only network where EVERY airplay is accounted for is 
the CBC (our version of
NPR and BBC) and you do see money for your ONE airplay on your 
statements (in my case, SOCAN).

i have been seeing some DIGITAL airplay monies recently (whoo-hoo - six bucks!)