Execute javascript on logout

I have this method, but the javascript never executes:

Protected Sub Logout(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.LoginCancelEventArgs)
        FormsAuthentication.SignOut()
        Session.Clear()
        Me.Page.ClientScript.RegisterStartupScript(Me.GetType(), "alertScript", "facebook_logout();", True)
    End Sub

Why?

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