Goodreads just announced the launch of a beta feature where Kindle using Goodreads members can:
... access and share your Kindle notes and highlights right from the book pages on Goodreads.com (PC or laptop). You can even edit your notes directly on Goodreads, and your changes will sync back to your Kindle book. By default, your Kindle notes and highlights are only viewable by you—you’re always in control. You can choose to share them with friends by marking any (or all) of your notes and highlights as “Visible,” and your friends will be able to view, like, and comment on them. You can also share them by sending the URL of your Kindle Notes & Highlights page to anyone you like—for instance, a colleague, professor, or classmate.
We already had the option of organizing our Kindle notes and highlights via Kindle.Amazon.com (we could even share them) and now Amazon is adding that feature to Goodreads.
Before you can use it, you'll need to tie your Kindle and Goodreads accounts, add the relevant books to your GR account, and start taking notes.
Coincidentally, if you don't want notes from a particular book to show up on Goodreads then you can simply not shelve the books - or simply leave the notes for that book as private (it's the default setting).
This sharing feature is currently available via the Goodreads website, but the announcement also mentions that Goodreads plans to add mobile support.