After a few days wasted switching between various sites, I have to pick one (or two, max) and stick with it, so I tried to determine which one had more chances to survive in the long run.
Data from Compete.com seems to say that GoodReads is winning the battle on literary social networking.Librarything, despite a huge head-start (years), seems to be in decline, even though attention data suggests that the remaining members spend a bit more time on the site than the average GR member.
Note how aNobii seems to be such a minor player, even though it's the dominant application in Italy and probably lists more books than its competitors; I wonder if Compete's data is skewed towards US/UK visitors. It has to be said that aNobii is showing more velocity though, so it will be interesting to see how things fare in a few months.
Shelfari is also in the mix, still below Librarything but growing fast. The site is very well presented, but I hated that it used Yet Another Format to import books (apparently, more oriented to listing the condition of your books, which I guess has to be expected from a site backed by ABEbooks), and I couldn't be bothered to try it.