I've curated my list of Facebook friends to be something like that. The list of real-life friends who make up a core part of my list of Facebook friends includes a lot of people from a membership organization gifted children, whose parents then form a mutual support network. This, to be sure, is a hard strategy to replicate exactly, but over time I think that your professional or other affinity groupings will help you find people who like to discuss books you like thoughtfully.
I get good book recommendations here on HN all the time, most recently a set of books about German history,[1] but we hardly ever have extended book discussions here.