Yes. Kindle is around 65% the ebook reader market in the US. Nook is about 25%. Apple and Kobo make up the rest. Kobo is more popular in Canada and in other international territories.
But, you just have to approach it from different angles. Don't attempt to compete with Goodreads, but rather build something better that works in a new way around people's desires around books.
Goodreads is just a review database, easy to innovate around that barrier.
Fair enough, but that is just ebooks. Isn't goodreads also useful for dead tree readers? Although I guess the bibliophiles on goodreads are more likely to have an ebook reader.