Not sure if these are of use but these are the free ones I have found.
MobileRead ForumsKindle Boards - Index
Kindle Forum, Kindle Social Network - Kindle Your Social Life --- We're Making Reading Cool Again
World Literary Cafe | Bridging the gap between readers and authors
There is also Jottify | A new space for writers to share, read and sell and Goodreads.com but these are not free.