Alright, well, I'm networking. Of the social-book-reading sites I've been looking at, Authonomy is definitely the most lively. It's made up mainly (or entirely?) of authors, and everyone seems happy to offer feedback and advice and be pretty nice about it in the process! Must be a British thing. The system is very reciprocal, and most people are being pretty upfront about that - you review my book and I'll review yours. I, however, don't have a book to contribute, but I'm still having fun offering up my opinions and seeing if they help. I've gotten a really positive response so far.
DailyLit I couldn't live without right now, but it still gets a mixed review. It has tons of classics, and tons of aspiring bestsellers, available to read free, and it'll send a little bite-sized read to your email every day at a specified time. The personalization options are fantastic, and it's so easy to get exactly what you want out of the function. Though I admit it's a bit weird reading Ulysses in 5-minute segments in the morning. But I've gotten further in reading it this way than I ever have before, so maybe there's something to be said for the method. On the other hand, the boards are pretty dead, and you get things like "27 comments, in 25 threads", where nobody is really talking to anyone. I think they should play up the book-club angle more, to get groups of people reading the same thing so they can talk about it.
Twitter I just can't figure out. It seems like people are making friends and forming lively communities, but there's no explanation of how this works anywhere on the site that I can find. Hmph.
Smashwords, Scribd, and Issuu I'm still working on... these sites all suffer a bit from being too geared towards writers and not accessible enough to people who want to be an audience. We'll see if I can figure them out any better in the days to come.