I love goodreads for their reviews. I also love the interaction with authors. I have many "friends" who are authors.
I use GoodReads, but not the social part. I use it as an handy inventory to have when I'm browsing books, to prevent buying duplicates unintentionally. The only social aspect I have contact with is that I like it on Facebook, so I get some general news that way, and I get notifications when others follow me. ( I feel vaguely guilty because I don't really follow others there, just enjoy the feeling that other people find my tastes interesting!)
I joined both Goodreads and Shelfari a good while ago, but Shelfari seemed more user friendly. The social aspect didn't seem very good, though, so I used it mainly to keep up with what I had read and wanted to read. Then, for some reason, I looked into Goodreads again and fell in love with it. I'm an active member in 3-4 groups with similar interests as mine, and I enjoy doing group or buddy reads, as well as simply chatting about books and life in general. Goodreads almost seems like the "default" book group now and I'm very glad Amazon purchased it.