You're forgetting how much credit card transaction suck. As I see it this is the main problem facebook could solve through sheer volume of use: microtransactions.
Granted this is about the worst way this future could come about....
Isn't this a lot like asking why Tyson invested in Beyond Meat when BYND is a threat to Tyson? Maybe you could call this defensively "investing in the future" of your industry (if this turns out to be anything at all).
if your company were under threat and you had the option to invest in your competitor at an early stage, wouldn't you? I don't think I agree that this is a threat to credit cards, but even if I did I think I would understand the investment.
Granted this is about the worst way this future could come about....