Yes — Amazon certainly has significantly more data, but Goodreads' data is deeper, because it's not restricted to items bought from Amazon. Goodreads has a much better picture of what I read than Amazon could ever get simply from my purchase history (even if they cross-match with bookdepository). Being able to mine the Goodreads data too should enable them to make even higher value recommendations. (Note: I'm not claiming this is why Amazon bought them!)