Goodreads is a great platform for writers and readers to meet. This site draws millions of visitors every month, and has a membership of more than 1 Million. As an author you can even get an author's page and a blog, all for free. Another important thing you can do here is announce a giveaway for certain days. You can choose whatever number of books you would like to give free, and to which countries you can choose. You will be surprised to see the number of entries who would like to win a free copy of your book. Most probably the winners will write a review also for your book, without even you requesting for it. In this way you are creating a buzz about your book. At present they will take only printed books for giveway.
This site gets a large number of visitors, and you have many options there to promote your book. Several groups of writers are there to suit your need, and is also a great place to learn about book promotion.
This is an old site and very popular.
Authorsden is another popular place where readers and writers come together.
Redroom is another wonderful place for the readers and writers to meet, and market your book.
For a small monthly fee you can announce your book here, which also has a good readership.
Books and Writers group and Ebooks, Ebook Readers, Digital Books and Digital Content Publishing Group are some of the very active communities in linkedin, which are useful for writers to promote their books.
Librarything is a very old site with a community of 1,500,000 book lovers.
It is comparatively a new site, and still it is becoming popular.
Jacketflap is an old and popular site with more than 200,000 authors, illustrators, publishers and other creators of books for Children and Young Adults, and promotes only Children's Books. So if you are a children's book author it is a great place to check.