We can easily create Facebook-Like textbox for leaving comments in an easy and cool way using jQuery, I am leaving an example below.

You can see an implementation working here

HTML

   <textarea class="comment_empty">Write a comment</textarea><br />
   <input type="submit" id="submit" value="Submit" style="display: none" />

jQuery

$(document).ready(function(){
    var submit = $("#submit");

    $("textarea").blur(function() {
        if ($(this).val() == "") {
            $(this).val("Write a comment")
                   .removeClass("comment_filled")
                   .addClass("comment_empty");
            submit.hide();
        }
    }).focus(function() {
        if ($(this).val() == "Write a comment") {
            $(this).val("")
                   .removeClass("comment_empty")
                   .addClass("comment_filled");
            submit.show();
        }
    });
});

CSS

.comment_empty {
   color: gray;
   height: 30px;
}

.comment_filled {
   color: black;
   height: 100px;
}