You're talking about the actual user-generated lists. Amazon also has the data on what all of their users have actually bought over time. (Hundreds of millions vs Goodreads millions.) That's what they use to create purchase recommendations, because it's higher value data than manual user-generated collections.
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!)
