In FB Mobile Browser ONLY - PayPal Add to Cart button when clicked asks users to log in and then redirects to their PayPal home instead of checkout for my site. This error has only been encountered in FB Mobile Browser, everything works in chrome on mobile and desktop.
Since PayPal relies on the use of "cookies", I'm guessing that your FB Mobile Browser cannot access those cookies which in turn will simply default you back to the main PayPal login screen.
