Came across this today and thought this was a pretty neat idea for book lovers' social networking.  Not only can they connect with people who like the same books, but they can connect with people while reading by leaving comments in the margins and by chatting with folks reading the same chapter/section as they are.  

Book Glutton -

"We set out to create a better way to read on-line; our goal was to make something different, engaging, intelligent and digital. The concept was born, as many good ideas are, on a crumpled cocktail napkin late one evening in 2006, and we’ve been working to build it ever since. Our intent: build an experience that is simultaneously a book group, a computer, and a book.

We believe firmly that people want to read, annotate and discuss, right there, immersed in the text. That’s the best time to talk about a book. We also respect the solitary side to reading: people should have the chance to tune out the community. We wanted it to be attractive, too; to be an experience. It was designed for the laptops people carry to their coffee shops, and meant for the network, not the desktop. Finally, it had to be something we’d want to use. Naturally we’ve got a list of improvements. Like any creative endeavor, we’re always seeing new ways to tweak it. And we’re open to suggestions! You can suggest features or give us general feedback."