I didn't know social networking sites for book lovers existed before a friend sent me an invitation to Shelfari. I was pleased. A typical list-making (over-analyzing) Virgo, I've been tracking the books I've read in an Excel spreadsheet for over 10 years. I was more than glad to have a way to share that information with friends, a way to see what they might be reading, and to answer any questions they might have about the books I've read.
So I signed up with Shelfari, and it was only later that I found there are several similar sites to choose from. So I did some digging around, and here are a few more I found:
If you want to know which is right for you, there's a fairly in-depth review and comparison of Gurulib, Shelfari, and Librarything at the librarytwopointzero blog. Also, here's an article/review at Publishers Weekly.
Google, never to be outdone, launched their own version in Sept 2007, called MyLibrary. You can read more about Google's version on the Wired Blog.
And if you're still undecided, a few other sites are available:
As for me, I'm glad I didn't research this earlier, before joining Shelfari, or I might've gone crazy with all the options. Overall, my Shelfari page is perfect for what I need it for, and it's got other features I don't even use (such as book reviews and book groups).
If you know of other sites, or would like to comment on any of the above, please leave it in the comments section.