Regardless of HN's opinion of Facebook (and regardless of my own for that matter) I predict that this thing will work.
This is only one thing my crystal ball showed me.
Facebook's presence in developing economies is massive. To the point of being synonymous with "The Internet" in a number of places. But they've had a nagging problem. People in these economies consume contents, but do not buy. Even when they have some buying power, access to credit cards is harder to come by. So they're basically seen as online leeches, and you simply fit them in the "expense" category of your media production. Also, due to their buying impotence they're almost immune to advertisement. Over the next few months it's all going to change. Multiple agreements will be signed with various financial institutions and probably more with various telecom in those regions, to allow people to load up their accounts with fbcoins and join the Great Internet Spending Frenzy. Basically turning them overnight into consumers, ripe for the picking.
I foresee big media producing companies in the developed world to be the first to take advantage of this (Disney, Valve, Netflix, YouTube, NYT, various online courses and certifications, etc). Shipping to those regions remains a challenge, so only soft goods for now. IKEA and Walmart will allow fbcoins, but just to be able to sell through their Facebook Store, oh I forgot about those. Anyways.
Next year, Google and Amazon will announce their own stablecoin.
The year after that, Google will announce that they're shutting theirs.