I previously wrote about LibraryThing, an online community for book lovers, where you can create a virtual library and share reviews, etc. While I really liked the idea of the site, I never fully integrated it into my online routine - perhaps because it stops being free after you add your 200th book.
That has changed with my discovery of Living Social: Books. Maybe it's because this site is free with no detectable strings attached, or maybe it's because of Living Social's seemingly more robust online community and modern user interface. However, I think the main reason I've been using it - religiously - to host my virtual bookshelf is its plug-in integration with Facebook. I really think the sites that will catch on in this era will be the ones, like Living Social, that allow you to use them as seamless parts of the "indispensable" sites into which you are already logged - or at the very least, ones that allow for the convenience of single sign on, such as Blogger's integration with Gmail.