How do FM radio stations know how many people are tuning in?

Here in the US they use ratings systems that survey random households. Participation of each targeted household is voluntary. From the results received, the ratings organization deduces assumed listener numbers. I don't see how it's very scientific. My online radio host shows how many people are listening when I am logged in, and sends me reports weekly and monthly, showing how many listeners I had, by country.

Hello Bliss Doubt!
Thank you so much. Yeah. You are correct. If we can get listener count report for FM, AM Radio like as online radio, then advertisers can be easily choose best FM and AM radio station.