I'm sure you're aware that one of the most sensitive issues when cloaking is the final target URL. Placing a pixel on the money page is like disclosing everything to Facebook. Logic suggests that if the pixel is placed only on the white page, Facebook will notice the gap, where traffic doesn't trigger the pixel.
Using the thank you page isn't a practical solution because we want users to hit the target link, not the thank you page. If the thank you page had both the pixel and the redirect, it would be vulnerable to the same "gap," unless it received traffic only from the white page.
The solution is to load the pixel in a temporary header, which gets discarded before the real landing page loads, such as through a redirect. The temporary header should parse the final URL, redirect, and then be left behind, where the pixel will reside.