I thought I'd post about this, because it was annoying the shit out of me and making me rage, and the Goodreads "help" about the topic isn't actually that helpful.
And if you follow the sharing advice on that "help" page, all you actually do is share a book cover with zero context into why you are sharing it - I think it does link back to your details about the book, but your FB friends won't know why you are sharing it (did you start reading it, did you review it, did you add it to your shelf, etc) and they probably won't click through without some amount of detail.
If you want to share your actual review you have to write it, save it, then on the book details page select "see review" (this is a link under your actual review text, next to the likes/comment links), and then select "Share on Facebook" from the review details page. That will also pop up a giant book cover, but at the bottom it will have your star rating and the first line of your review.
It's a way shittier experience than the old way (way too many clicks) and the UI is still awful because it's still not giving enough prominent context IMO (and displays WAY less of the review), but I know a lot of my FB friends see my reviews and then go to Goodreads and engage more with me about books because I'm sharing the reviews in two places (they don't hang out on Goodreads daily) so I don't want to lose this feature.
I am glad to see that someone else is bugged by this. UGH! I used to be able to just share away, and lately it was giving me a huge pain in my neck...not being able to share really at all. It used to be automatic, just to share a book. Now I can't even hook it up to do that.