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.