I like Librarything and similar sites a lot. I can only afford to buy one or two books a month at best, so most of my reading comes from the library, and it's a nice way to organise all the books I've read, but don't own.
I'm not about to pay a site just to get to list my books there though, and any active reader would run out of space on a "free" Librarything account very fast. On a positive note, Librarything is quite useful for looking up new authors and books. I like how it recommends similar books, and that "how much would I like it" rating meter actually tends to be moderately accurate.
After I filled up the free space you're allowed on Librarything, I started looking for similar sites and found Shelfari. It doesn't have all the fancy features that Librarything does, but in my opinion the design is much nicer. Instead of appearing as a list, your books appear on a shelf. It lets you customize the shelf to your preferred look as well. It's pretty similiar to Librarything in other aspects, and also lets you choose covers, write reviews, add notes, dates, etc.
I don't know if you can link your booklists to Facebook from Librarything, but you can from Shelfari. Check it out if you like staring at neat and organized rows of books