It is not possible to utilize PayPal in Facebook's browser, which is noted in your documentation here:
Checking out via Facebook's in-app browser is not currently supported. This is due to Facebook's browser not supporting tabbed browsing, which prevents the PayPal checkout flow from displaying correctly.
Can you get a fix for this on your roadmap as soon as possible? Non-recurly PayPal functionality works as expected there, and the alternative is a custom integration (which is not advised by Recurly).