Do you remember when people made websites to solve problems and build community, instead of to make a profit? You can always just shill for amazon using affiliate links, everyone wins.
The value of a social network is directly proportional to the number of edges in the network graph (I.e., SocialNetworkX might be better than Facebook, but if none of your friends are on it, it’s worthless). For Goodreads, couple that with the vast amount of book metadata they have + Kindle integration, and any startup would have a long way to go to even reach parity.
Scalability + data aggregation + user adoption == lots of funding + a clever business model + a significant reason to switch from $DOMINANT_COMPANY + a good bit of luck.
That requires huge financial resources, and thus, a solid monetization scheme.
Making money (and this site will take a lot of money to build, even if it's just to break even) via Amazon -- while competing with an Amazon property -- is not something that is going to work. I frankly think this is the central reason there's no goodreads competitor: how to make enough money to break even when Amazon becomes your enemy...
