I love http://goodreads.com very much. It has measurably improved my life as a reader. I have read authors I wouldn't have read without it, books from those authors I would have ignored, and keeps track of what I read, am reading and will read.
What it has never been for me, is a social network. I would be about as happy with it if I knew noone else on the site, if it were just me and a bazillion strangers whose taste I can leech off.
Sure, I have a few friends there nowadays, but I hardly ever do anything "social" beyond accepting requests and posting reviews which I have no idea if someone reads.
I love Flickr where I put most of my pictures (soon: all of my pictures). It's cheap and I can upload an almost infinite amount of pics there, and I can share them with friends and family sometimes (by reposting them to facebook).
They were even kind enough to store the pictures I uploaded as a free user until I paid for the space to store them 5 years later.
I love Twitter because it's a place to post short things that don't deserve a blog post, to chatter with friends and not-so-friends, to know more people, and to waste some time every day.
One of those things is not like the others. One of those things I use for its social features, the others I use for other reasons, and I don't really care about them being social or not.
I think nowadays, for a social network to succeed, it has to cater to the antisocial, at least at first, when you know noone there. I don't go to Flickr to debate. I don't go to Goodreads to chat. I go there to put pictures and keep my books straight. And that's what kept me there long enough to meet people.