However, I didn't have a personal LibraryThing account because several of my friends were already on Goodreads and had invited me to join them there. And that's the thing with these social network sites--they're really only fun if they're social. So, you go where your friends are, whether or not it's the best resource.
Which is not to say that I necessarily think LibraryThing is better than Goodreads. At the moment it would be like comparing action-adventure to chick-lit. They each have their benefits and limitations--and to some extent it depends on your taste and what you want to get out of the experience.
For me, the difference is currently in use. I'm more familiar with Goodreads as a social experience. I can log in at any time and see what my friends are reading (assuming they keep their collections updated), read their reviews, and (possibly the most fun) compare collections and ratings.
LibraryThing, while it offers a range of social experiences, I've only experienced the cataloging end. I love the fact that the cataloging comes from legitimate sources and that both tagging and creating collections (what Goodreads calls 'shelves') are allowed. That provides a lot of flexibility in how you can group items.
My current feeling is that LibraryThing is for people serious about organizing their book collections (which may be why "Librarians Who LibraryThing" is one of the largest and most active groups) and that GoodRead is for people who like to read, but are also more interested in the social aspects of sharing books. Maybe that will change over time as I use both and get a better sense of what each can do.
My favorite find today was the "Name that Book" group--in which members help others figure out the name of that book when you can only remember a snippet of plot, color of the cover, or so on. It looks like a highly active and friendly group--and a wonderful resource for front-lines librarians.
