I've really enjoyed exploring Library Thing. I added 26 books and rated each one. I'm always looking for good books and new authors to try so I really like the recommendation feature. It's just like getting recommendations from Netflix when you rate movies you've seen. Like Netflix, you can see the list of recommendations and the books that you've read that are similar. It gives me options I would have never chosen on my own.
I also like looking at the members that have a similar library to mine. I visit their site and see what books are the same as mine and what books they've recently added. It gives me new ideas for books I might like to read.
The review section is helpful too. I always like to know what other people think of a book before I read it. You just have to watch out for "spoilers".
I searched through different group options but didn't find any I would be interested in following. I like such a wide variety of books and it seems like most of the groups are focused on one author or genre.