Launched a couple of weeks ago, iDreambooks is a new site dedicated to books and especially book reviews. I am active on Goodreads - interactive and with many interesting authors around - and Shelfari - a big library and many people, with a minimal interaction between readers.
As far as I realized in the last minutes since I created my own account, on iDreambooks for each book, you have the reviews published by now in the media. When you are interested in a book, you enter the title and you can see the aggregated reviews and recommendations. If you've read already the book and you have your own opinion, you are free to vote the review.
Regularly, there are giveaways available, but as far as I saw by now, you should be mostly from the US or Canada or the UK for being allowed to enter. Hope this situation will change for the sake of the European readers.
It is a site for both readers and writers. The latters can create their own giveaways and set up their profiles. As a simple reader, you can build your reading list, up to the latest preferences.