Good Reads is a social site for book lovers to share the books they love with others. According to the website, you can view the bookshelves of others as well as their reviews and ratings. You can also post your own reviews and catalog what you have read, are currently reading, and plan to read in the future. Beyond this, members can also join a discussion group, start a book club, contact an author and even post their own writing.
In browsing the main page, it looks like some books contain content that I would not want to expose younger students to. I think that this may be a great site for adults but would ask that you explore the site carefully before telling students to sign up.