After beating my head against this for a couple hours a pretty stupid solution occurred to me and it ended up working.
Forget editing woo pages directly, simply go into the Pages pane in the WordPress control panel and select the – Order Received page. In the edit window, choose the “text” tab (rather than the “visual” tab) and above [woocommerce_thankyou] , paste the FB tracking code. Viola.
Stupid, right?
