There should be a messaging system on the site, so that you may contact people who have similar reading interests. Send people recommendations, like their lists, etc.
Hi Matt, We have added improved ways to search for other users in Riffle. From read.rifflebooks.com click on the People Tab at the top of the page. You may search for people based on category, or find Facebook or Twitter friends (depending on how you login or the account you connected). We will consider a messaging service – perhaps something similar to twitter where you can only message people if you are both ‘Following’ each other. Thank you for your feedback. Best, Marquina and Riffle Support