Facebook issues participate in the crypto market to possess personal information, I think they will not need to do so to get our nonsensical information.
They participate in the crypto market just to get more partners to expand and develop their payment services.
The purpose is behavioural analytics used to predict and influence future purchasing. It's an incredibly lucrative field. Say someone wants to sell a sports drink. Instead of wasting money by blanket targeting everyone through say a TV advert, they can get an ad to pop-up on your phone when you've just left the gym. They know there's a shop just around the corner that sells their drink, they know you're thirsty because you've been exercising, they know you have a history of buying other drinks from that shop at that particular time of day. Their model indicates that you are attending the gym more often than you used to. You're trying to engage in a healthier lifestyle. They then sell this information on again to a client company who produces juicers and smoothie-makers, and another company that sells bicycles, perhaps they'll offer you 10% off a new bike, but only for a limited time - and they'll send this offer to you at 8pm becuase they know this is when you're tired and more likely to make impulse purchases.
That's the point of it. Companies pay a lot of money for this sort of advantage.