Well, we still have the G-rating so far LOL.
Like me you have probably got a lot of unintentional spam about Shelfari. My advice is, don't do it. But if you must, do some research first.
Shelfari advertises themselves as the first book-based social networking site. They aren't.
Shelfari has spamming your gmail contacts as the step right after signing up, making mistaken use of it common. Besides, that's asking people recommend a service they haven't even used yet.
Shelfari makes it easy to import your records from a competitor but impossible to export them back again.
Shelfari uses only Amazon's catalogue when competitors use multiple sites. Quite by coincidence Amazon is a part owner of Shelfari. (Yeah right, expect 'monetising' strategies very soon).
On the plus side they have a pretty 2.0 website, they are sparkling new and have a lot of (largely accidental) cheerleaders. Are we seeing the parallel here? ;)
If you happen to want to be part of a book-based social networking site you should... 1) think about what you want in such a site, 2) discover what sites are out there and 3) pick the best one... if you decide that is Shelfari please keep your finger off the INVITE button!