When Goodreads started out, Facebook's API was much more open. After you authorized Goodreads they'd have all of your Facebook friends and would proceed to spam them asking them to signup for Goodreads. The openness of Facebook's API was probably very critical to their early growth.

These days though, the FB API has been locked down and basically can't be used for growing your userbase anymore. Any new startups in this space won't have the social graph advantage that Goodreads did. Sad.