Most people I know use goodreads. If you're one of the few people who is not familiar with it, you're totally missing out.
Goodreads is to books what Ravelry is to the fibre (yeah, I'm Canadian) arts community.
Goodreads is a community of readers and writers interacting and participating. You can use goodreads to keep track of your books, both read and to-be-read, categorize them on your virtual bookshelves by whatever ways make the most sense to you. You can give each book a rating out of 5 stars, and post reviews to share with the community.
Also within the community are different book clubs you can join. I'm not one for book clubs (ok, I really view them with as much pleasure as I view being infected by the plague, if you must know), but I think if I were forced at gun point to join a book club, I'd join one on goodreads because I can discuss books in my PJs without having to venture out of the house and drink whatever the heck I choose instead of whatever is served. Truly a fabulous option for a non-people person like me.
Another great thing about goodreads is that there are at any one time, hundreds of authors having giveaways of their books, some are even ARCs (advance reader copies)! I've won a book before. It was good too; I gave it to my son to hone his writing when I was done with it.
And if you join goodreads, pleeeease friend me! I love to see what others are reading even if it's a genre I have no interest in. I'm just a bibliovoyeur. Hey, I invented a word!
"If there's a book you really want to read, but it hasn't been written yet, then you must write it." ~ Toni Morrison