Use this script for facebook logout. It works on my site
<form>
<div id="fb-root">
<script language="javascript">
window.onload=function()
{
FB.init({ apiKey: 'b65c1efa72f570xxxxxxxxxxxxxxxxx' });
FB.getLoginStatus(handleSessionResponse);
}
function handleSessionResponse(response) {
if (!response.session) {
window.location = "/mysite/Login.aspx";
return;
}
FB.logout(handleSessionResponse);
}
</script>
</div></form>