How do I change the singular/plural on "comment" to "comments" on Facebook's number of comments?

Using the fb:comments-count tag by itself, you can't. What you need to do is get the number of comments into a PHP variable first and then print the correct phrase depending on the value of that variable. You can retrieve the number of comments using the PHP SDK, FQL, or the Graph API. Then, one way to print what you want:

 <?php $comments = getCommentCountUsingGraphAPI(); if ($comments == 0) {    echo "No comments"; } elseif ($comments == 1) {    echo "1 comment"; } else {    echo "$comments comments"; } ?>

But it's a lot easier to compromise and just modify your presentation a little to avoid the pluralization issue entirely:

 <span class="comment-count">      Comments: <fb:comments-count href="<?php echo get_permalink($post->ID); ?>"></fb:comments-count> </span>

Or:

 <span class="comment-count" title="Comments">      <fb:comments-count href="<?php echo get_permalink($post->ID); ?>"></fb:comments-count> </span>

An alternative to the straight PHP method would be using the ngettext() function.

<?phpecho ngettext("%d comment", "%d comments", $comments);?>

I combined John's solution with 9bugs tip with FQL and it works great. Add this to functions.php:

 function fbCount($url) {    $base_url = "http://graph.facebook.com/fql?q=";    $query = "SELECT like_count, total_count, share_count, click_count, comment_count FROM link_stat WHERE url = '$url' ";    $new_url = $base_url . urlencode($query);    $data = @file_get_contents($new_url);    $data = json_decode($data);    return $data->data[0];}

Then add this to where you want the comment count to show in content.php:

    $url = get_permalink($post->ID);    $fb = fbCount($url);     if ($fb->comment_count == 0) {        echo '<a href="' . $url . '"> leave a comment! </a> ';     } elseif ($fb->comment_count == 1) {        echo '<a href="' . $url . '"> 1 </a> comment';     } else {        echo '<a href="' . $url . '"> '. $fb->comment_count .  '</a> comments';     }