I'm on both Facebook and Twitter, and I use them to *chat*, not to sell. Sometimes the chatting leads to sales, or it leads to promo opportunities such as when someone started a review blog specifically for GLBT YA fiction and contacted me on Facebook to ask whether she could do a feature on me because she'd seen me talking about my GLBT YA books there. Not *promoting* them, per se, but mentioning that they existed, talking about how important I think it is to have GLBT characters in young adult, etc. That exposure in turn led to an increase in sales of the book she featured.
On both sites, Twitter especially, I also share/retweet tidbits from others, whether it's a quote I like or a new release announcement from another author or something like that. I share other people's good news as well as my own.
I can't wrap my head around Goodreads. I've been trying for over a year, and so far all I've managed to do is add my books to the bookshelf of the M/M Romance group (and a friend of mine actually added some for me as she tried to push me to do it myself), and hang out on the YA GLBT group. Both of my pen names have author accounts there, but other than existing, I don't really know what to do. The groups I've joined, other than the YA GLBT one, are huge and overwhelming and confuse the heck out of me; I don't rate books because I'm picky as hell and don't want to piss anyone off by rating them lower than they would like. So I'm not quite sure of the use of Goodreads as a promo tool, though I know some authors have a lot of success interacting with readers there. (Oh, and I've done a few book giveaways there, which has boosted my visibility.)