Goodreads is free and runs on ad revenue, whereas Library Thing is ad-free and free up to 200 books, after which you are asked to pay /year or for a lifetime membership.
Probably as a consequence of the differences outlined above, Library Thing has 473,080 users while Goodreads claims over 1,000,000.
This despite Library Thing being founded August, 2005 and Goodreads December, 2006.
For the social aspects (accessing my friends’ opinions when I need them) the population difference makes Goodreads the victor – vast mobs of my acquaintances signed up for Goodreads (many of them, strangers to each other, at exactly the same time, in fact.) The social exchange Goodreads emphasizes is that of opinions, and therefore there are a lot more reviews on Goodreads in general, even if there aren’t any from your friends on a particular book.
) I keep up on Goodreads though, because the social aspect book-cataloging needs.
Update, 9/7/2008: As I recently discovered while browsing a used bookstore in Mountain View, Library Thing is quicker and easier to use on the i Phone, which surprised me.
Its customizable home page, recently revamped, gives pride of place to a search-box that searches your own library of books.
By default, the next item down is your recently added books.
Library Thing has a great book-adding interface that allows you to type the tags once for one group of book-adds and integrates a barcode scanner seamlessly.
On Goodreads, it’s harder to determine exact numbers but I believe 9173 people have entered it, 7272 have given a star rating, and about 700 have entered at least a one-word review. I may sound elitist here, but I’m not running for office, so who cares?
Probably as a consequence of the differences outlined above, Library Thing’s smaller population is more serious about books.
*Updates, 2/16/2009: Goodreads has added multi-author and customizable role support to their book data, although it is not as integrated and easy as Library Thing’s.
They have added an experimental mobile version of the site, which loads more quickly.