Findings

I have a problem with the FB button share, where I have this code

Share
<script src="http://static.ak.fbcdn.net/connect.php/js/FB.Share" type="text/javascript"></script>

The error message with internet explorer is:

FB.loader is null or not an object.
http://static.ak.fbcdn.net/connect.php/js/FB.Share
FB.Share                                               line:15
Code:0                                                 Char:1

or in firebug console:

FB.Loader is undefined
        FB.Share()FB.Share (line 15)
        [Break on this error] FB.Loader.onScriptLoaded(["FB.Share","FB.SharePro"]);

Solution:

On looking the code, there is a FB.Loader is missing. You have to add another script to load "FB.Loader" function.

Share
<script src="http://static.ak.fbcdn.net/connect.php/js/FB.Loader" type="text/javascript"></script>
<script src="http://static.ak.fbcdn.net/connect.php/js/FB.Share" type="text/javascript"></script>