“By Dec 31, 2021 the total volume of payments denominated in Facebook Libra (or any replacement introduced by Facebook) will NOT exceed $1.8 Trillion in a three month period.”

Just the nature of distribution on Facebook is that it Libra will get Billions of USD in transaction volume essentially overnight. Tether does about $20 Billion in daily volume as of June 16, 2019. For Libra to meet or exceed that would constitute a significant success, and I predict that Libra will not get there.