I used to work at a library supplier and catalogue the new books for public libraries and when I saw a book that I wanted to read in the future I would covertly open up the Goodreads app on my phone, go to 'scan' and scan the barcode and then add it to my 'to read' folder. Then I'd put holds on them at the library, naughty me! Sadly I no longer work there...
With both social reading sites I really like that I can keep a record of the books that I've read and the dates I read them and it's nostalgic to look back over the list :) I don't like reading reviews of a book I'm about to read though because I want to read it with an open mind and some of the reviews are overly critical I find. So I read them after I've finished the book and often see the book in a different light, one that I hadn't previously considered. I don't use many of the other social features, but may do in the future. I've only been using the Goodreads app since the beginning of the year, so not many books in my collection yet.
I've got quite different reading habits to most people. I pretty much stick to fiction, written in the past 10 years, and often translated into English. My appetite is for the foreign novel - it's like travel for the mind while sitting on the sofa and I'll never tire of it as long as it's well written. If it's good enough to get translated then it usually has merit. At the moment I'm really into Japanese and Chinese writers, although I'm reading a Nigerian writer now. Why is there never a section for international fiction in the library :(
I've gotten off topic. I read the excellent Library Thing vs Goodreads by Amanda Nelson and she discovered that LibraryThing came out better than Goodreads on almost all criteria, concluding that LibraryThings was the more serious tool and Goodreads was the more social. LibraryThing doesn't have an iPhone app and I need to have access on-the-go without requiring the more serious features, so Goodreads is fine for me, but I still (sometimes) maintain my LibraryThing catalogue too. It's could take over my life, the amount of detail you can catalogue in and the other fun functions.
Oh one more thing. I've been trying to make the Goodreads widget thingy work on my blog. Firstly it was really difficult to locate the widget section on their site, and then it doesn't update. Will look into it. Feel free to add me as a friend on Goodreads, especially if you read international fiction.