This worked for me:
<a href="<?php echo esc_url( wp_logout_url( $_SERVER['REQUEST_URI'] ) ); ?>">Log out</a>
I added it to the /sfc-comments.php in the sfc plugin folder
"<a href='#' onclick='FB.Connect.logoutAndRedirect(\"<?php echo esc_url( wp_logout_url( $_SERVER['REQUEST_URI'] ) ); ?>\"); return false;'> <?php echo addslashes(__('Logout', 'sfc')); ?></a>"
I hope this helps, here is the link where I found this info:
http://codex.wordpress.org/Function_Reference/get_permalink
