Reticulum supports multiple interfaces to transport data, TCP is just one of them. Other are ethernet, packet radio TNCs (think ham radio), LoRa, stdio/pipes, I2p, etc. More details on some of the supported interfaces http://reticulum.network/manual/interfaces.html
You can get a rough estimate based on unique IPs hitting the RSS feed. Moreover, some of the online feed readers report the number of subscribers of your feed as part of their User-Agent. An example from my blog logs: `"Feedbin feed-id:2688376 - 9 subscribers"`
For receiving FM radio a lot of them do, definitely most of the feature phones that have a 3.5mm audio jack (the audio cable doubles as the antenna).
Some phones released around the 2010s also had FM transmitters, for example the [Nokia N8](https://www.gsmarena.com/nokia_n8-3252.php). The use case for these was to be able to listen to the music you had in your phone via your car stereo, since AUX ports and BT sharing wasn't common back then.