Here's an alternative to the accepted answer that works in the current (2.12) version of the API.
<a href="#" onclick="logoutFromFacebookAndRedirect('/logout')">Logout</a>
<script>
FB.init({
appId: '{your-app-id}',
cookie: true,
xfbml: true,
version: 'v2.12'
});
function logoutFromFacebookAndRedirect(redirectUrl) {
FB.getLoginStatus(function (response) {
if (response.status == 'connected')
FB.logout(function (response) {
window.location.href = redirectUrl;
});
else
window.location.href = redirectUrl;
});
}
</script>