As a high school librarian, I haven't had occasion to use Shelfari with students under age 13, but have you considered other options or platforms for this activity? Could you have students join and post to a protected wiki or blog instead? Or how about using safe, secure Edmodo as your social-networking platform?
I LOVE Shelfari--I have a bookshelf on the library homepage and it starts book conversations. I like good reads too but that one is blocked by the filter at school. The whole have to be 13 years old thing is really getting annoying. I teach 6th graders everyday in a research class and I so wish I could get them on Shelfari for summer reading. I shall check out Edmodo now.
We use moodle and wikispaces so maybe having a summer reading page on the wiki would be a solution for now.
Thanks for posting the idea.