This isn't a good analogy. IMDb was and is literally the best source of movie data anywhere. It is more useful to think of them as Wikipedia for movies since the community maintains the info and includes data that isn't available anywhere else.

Goodreads is the largest community of book readers in the world and so they would be able to provide their tools for book groups and conversations about books that Amazon is sorely lacking.

That isn't a good analysis. I have found wikipedia itself to be the best source of movie data. For all titles I've searched (and not always mainstream or US releases), wikipedia has a better summary, a better plot description, more release info, and a better page layout. Perhaps it doesn't list all the production details such as who was the key grip, but you can easily jump to all the major cast and producer/director wiki pages. It just seems more complete and easier to read. Imdb now seems too specialized, like it's for the movie industry itself.

That might be precisely what the OP meant by 'best'. To me the distinction they make seems important and not inconsistent with what you've said.

Of course, Amazon's purchase of IMDB took place several years before Wikipedia even existed.

I'd compare Goodreads to Foursquare. I mostly use it for my own purposes of tracking what I'm reading (similar to checking in at a restaurant) and occasionally see a recommendation from a friend I'm following (similar to restaurant tips/check ins).