Then, respond to them. Seems simple, but there are plenty of sites that don't make it easy to get a human response. Build that list. If you run a site that talks about purple orchids that bloom only on the third Thursday of October, you'll get a fairly narrow list of devotees. Just make sure you capture their names with an email opt- in list.
People like to belong to groups with similar interests.
I email them every two weeks to let them know what's coming up on the site and ask for feedback. I've made friends with people around the world that I've never met and never talked to on the phone. Make sure they can opt out. Make it easy for people to get off your list. People's moods change, and their interest in your site can change, too.