I've been a member of Library Thing since 2006. Haven't really followed through - so little time and so many things (hey, that could be a new T-shirt slogan!). I have an account & I've listed a few books. I just added their new widget to my blog - I love widgets - they would easily become another addiction. Another addiction is something I don't need. At least not now.
Many of the members of The Betsy-Tacy listserv are members of Library Thing or GoodReads which is similar or both. Right now I'm so busy selling books that I don't have nearly the time I once had to read books. There is a distinct similarity to being an unread bookseller and barefoot cobbler. I can easily see how both could become a happy obsession. MySpace for the literate and for those who can write a complete sentence. As an aside, have you noticed how many spelling & grammar erors there are on the typical Myspace page? Myspace is not a good advertisement for the success of American Education.
I'm also on Paperback Book Swap, which is a fun way to find homes for unwanted books and replace them with wanted ones. Learned about that one via OnLine Booksellers, one of the many Yahoo Groups I belong to (Groups and Listservs are a long standing addiction). It's similar to Library Thing, though it's more of a swap site than a personal library catalog site.
They both appeal to librarians - one allows us to catalog our own books (would that it was that easy to catalog at work) the other allows us to put books into the hands of readers.
Once again, so many groups, so little time!