You have several variations listed, but the proper format is this one:
<asp:LinkButton ID="lb_LogOut" runat="server" CausesValidation="False"
Text="Logout" Visible="false" OnClick="lb_LogOut_Click"></asp:LinkButton>
or better yet, this one:
<asp:LinkButton ID="lb_LogOut" runat="server" CausesValidation="False"
Text="Logout" Visible="false" OnClick="lb_LogOut_Click" />
This should work, once you remove the "visible=false" property.