Create a Facebook page for your book club. Students have to join the page and then can post questions and comments about what they're reading as they're reading.
Create a Twitter handle for your book club (@thebookbunch) and encourage students to tweet about what they're reading. Make sure they include the title of the book with a hashtag (#eleanor&park) so you can search and organize their responses. Tweeting allows readers to react immediately to what they are reading. Be prepared to reply to A LOT of tweets. But it's fun!
Give your students the option of writing a book blog. Blogging is a popular form of writing these days and if your students write about what they're reading, they are thinking critically about the text, summarizing, asking questions, formulating opinions, and WRITING! I suggest using Google Docs or Wordpress.com for blogging.