Not all channels are active. Just because users are there, doesn't mean they are at their computers or that they are watching mIRC or that they want to chat. Try a channel where you see a lot of people chatting. If your text is showing in the channel, people can see it unless you have a script blocking that (unlikely, but possible). You can always open mIRC twice and connect to the same network/channel and see if you see the text from the other mIRC. If so, then others definitely can.