system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do radio stations know how many people listen to them?
gpt
Media ratings services. Companies in a given market make random phone calls to people in a given area and ask them what stations they listen to (and a whole bunch of demographic questions). Based on the answers given by the sample group they are able to extrapolate how many people are listening to what. 
With the rise of Cell phone and the decline of land lines these surveys have changed. Simple direct mailing with a cash offer (mail this back and we'll send you $5) are more the norm. 
Social media is also used to track a given audience. Ever notice how most radio stations are begging you to like them on face book?