I have never really looked into MySpace or Twitter so I feel I cannot provide an opinion on these sites but I do have profiles for LibraryThing and GoodReads. I love love love LibraryThing for keeping track of my books, reading reviews and the EarlyReviewers program among a few features. I post reviews on GoodReads and I have a couple "friends" I chat with and authors that I follow (Lisa See, James Rollins) on this site.
I use networking sites that have book-related features. MySpace does not interest me because I already have a blog and it would be too similar. Twitter would be the least attractive site to me because when I am communicating with others I like more depth than just 140 characters! I connect my blog, GoodReads and LibraryThing together by always linking back to my blog in my reviews. I keep ChaptersIndigo Community and Facebook very separate from blog/LibraryThing/GoodReads...not sure exactly why but I suppose I like the anonymity of these sites whereas I provide a more public face on ChaptersIndigo and Facebook.
Hmm...only one site?? I guess LibraryThing. I only started keeping in touch with relatives on Facebook so it would be easy to revert back but I must have a place to talk about books and I believe LibraryThing is the most comprehensive site.