Reader communities are a specialized type of social networking site where readers and authors network, and readers recommend books to others. For authors of fiction and children's books, reader communities may be even more useful than the more general social networking sites like Facebook.
Many reader community sites allow authors to set up a profile page to promote themselves and their books. Some sites allow users to create a "friends" network and join groups composed of readers and authors of a particular genre, and some offer promotional opportunities to authors.
Look over the various reader communities to find those that are the best fit for your book and your audience. You may want to set up a profile on several sites and visit them occasionally, but it's probably best to focus your time on just two or three sites.
Goodreads claims to be the world's largest social network for readers. The site has 1.8 million members who have added more than 41 million books to their bookshelves. Authors can promote their books in a variety of ways.
LibraryThing has 600,000 book lovers who have cataloged 35 million books. Registered LibraryThing authors can promote book readings and events, participate in author chat, offer review copies, and add photos and information to their author page. See this page for details.
Authors can build profile pages on AuthorsDen and interact with readers. Nothing Binding is geared toward independent publishers. Shelfari is owned by Amazon and it's popular with Facebook users. Red Room is another popular site.
JacketFlap focuses on children's and young adult books and Big Universe is devoted to children's picture books. Amazon.com offers groups (called communities) and forums (called customer discussions). Customer discussions appear just below the customer review section on book detail pages.
Links to additional reader communities can be found on this page. For more information about promoting yourself and your book through social media, check out The Savvy Book Marketer's Guide to Successful Social Marketing.