It's not safe to get back in the water yet: I just discovered, via Read Street, another social networking site, FiledByAuthor, marketed as for authors but I think readers might enjoy it too, despite a plethora of existing social sites and online groups for readers. The hook for readers is that you can "connect with your favourite authors" - but although J. K. Rowling has a page, I am not sure what "connecting" with it does in that regard (James Patterson and Stephanie Mayer have pages that are featured prominently, also). FiledByAuthor could, however, be a way to connect with other people who like reading a particular author's books. Some categories include Women sleuths, Police procedurals and Hard boiled - or you can search by an author's name. Here's Stieg Larsson, for example.
What's in it for authors? Well, apparently you can have a profile, promote your work, "add content" etc. I am not sure if that is significantly different to social sites such as Book Place or Crime Spaceon Ning, but if you are an author I suppose it is another venue for getting known. (More about the site is here.)
I think I will check out FiledByAuthor as a reader, to see if there is any crime-fiction discussion on offer. I will let you know if I find any! I have set up a group for crime and mystery fiction readers, so please join it if you want. (I have not put anything in it yet, but contributions are welcome.)
There is also an associated blog, FiledByBlog, which looks to be a useful source of publishing news.