php callback appending an extra 0?

I am trying to write a simple WordPress plugin, which updates a custom field value and increments it by one, and then echos the result which is handed back to my javascript code. For some reason, the data returned by the callback function always has an extra 0 appended to the end.

function like_post_callback() {
    $clicked = $_POST['clickedOn'];
    $id = $_POST['postID'];
    if($clicked == 0) $key = "like";
    else $key = "reallyLike";

    $prevLikes = get_post_meta($id, $key, true); //true, so we only return a single value
    $likes = 1;
    if($prevLikes == ""){
        add_post_meta($id, $key, $likes);
    } else{
        $likes = $prevLikes + 1;
        update_post_meta($id, $key, $likes);
    }

    echo $likes;
}

And here is what is calling it:

$.post("<?php bloginfo( 'wpurl' ); ?>/wp-admin/admin-ajax.php", 
{ action: "like_post", clickedOn: which, postID: "<?php the_ID(); ?>"}, function(data){
    alert(data);
    $("#" + clicked).html(text + " (" + data + ")");
});

Everything works, I just have an extra 0 in data for some reason, and I don’t know where it is coming from.