system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do radio stations know how many listeners they have?
gpt
Surveys, (primary Nielsen), and user engagement - it's one of the reasons they run competitions on air. Only a tiny fraction of listeners respond, but the size of that fraction is relatively consistent. 

Edit: As mentioned in several other comments below, some people have listening devices (PPMs) that send data back to Nielsen. A range of people are selected to cover different demographics. 

Internet radio is over a two-way data connection anyway. 

Corrected spelling of Nielsen.