Hello Andrew, are you trying to send Offline Conversion back into Facebook from Leads that originated from Lead Ads only? Only a small portion of our Leads come through Lead Ads. However, we are trying to push all Offline Conversions back into Facebook hoping that they'd match on email address or phone number. I think that the Facebook Offline Conversions only works for Lead records with the Facebook ID.
I just posted a related question. Let me know if you found your answer some other place.