Facebook has so much money that one can't wonder what their angle with this. I can only imagine they want to know exactly what you spend money on so they can better target ads.
They've certainly tried payments and money-transfers before. This isn't entirely new territory. It's just very country-specific. They get hit by high transaction fees. No one wants to type their full credit card and address info, then deal with required two-factor authentication in Europe, etc. And nothing is worse than having to forward someone from your app to an unreliable slow desktop-targeted bank page which then sends you an SMS/email that you have to type back into the bank.. etc.
FB is actually right. The credit card system is straight broken from a user-experience point of view. It wasn't created with computers, mobile, or basic security in mind. It's just band-aid on top of band-aids since the 70s. And obviously cash doesn't have much presence online in western countries. FB struggles to get people to buy things through their apps. They desperately want to fix that.
Crypto currencies just work better at a fundamental level with mobile payments. It has always just been a lack of institutional support to get them supported. VISA/MC have probably been looking for a way to leapfrog bitcoin/etherium for some time now. I'm not surprised they jumped on board when a bunch of real technology companies decided to leapfrog them. Otherwise, they'd likely be in for a world of hurt in a few years if something like Apple/Google Credit launched their own without them. It's not like people love VISA/MC as companies. A huge number of people would jump ship so fast if there was literally any other option.
What is the quote again from John Wanamaker "Half the money I spend on advertising is wasted; the trouble is, I don't know which half."
Thus knowing more about their users and what they buy and the ad is successful 1 day, 1 month, 1 year in the future allows Facebook to get paid more.
The 2nd order effect is that by making more money for the same ad, while your competitors would make less money from the same ad...you can either sit on that profit.
Or you can use your market share and guaranteed profit to make ads less profitable to competitors and this creates a positive feedback cycle where Facebook Ads get more and more market share but other competitors and platforms of Ads gets less and less, which in turn actually allows Facebook to charge more per Ad in the long run (but in the short run Facebook may want to decrease the price per Ad.)
People say a lot of things on the internet, and visit a lot of places. Trying to infer who they truly are and what they truly like from that is actually hard: signal to noise ratio is not great.
But when the time comes to actually spend money, peopl really tells you who they are.
It's the digital equivalent of "actions speak louder than words".