This is the same as blocking content on your own forum or comment section on your blog. Yes fb is huge, but still just a website, and one with fading popularity.
Blocking ips on a network level is different.
People use FB because other people use it. There's a lot more complexity, and algorithm fuled habits. But in the end, FB provides the service of communication and content recommendations. Using that attention, it can sell ads. Without that willingness to give attention, they can't sell ads. There are no significant hurdles to starting a social media site.
Credit card processors facilitate payments from one group to a different group. They aren't an endpoint, they are middle men. They don't need to court the attention of users, they are in a position of power it where they can interfere with the lives of others, and have formed a coalition with a total monopoly over the digital trade of money. Good luck starting a competitor while attempting to shun PCI compliance.
If I never use FB, I can still interact with friends, family, buy and sell ads. If I never use a credit card... I've been cut off from the vast majority of the things that I would buy.
It's reasonable for different rules to apply to groups with vastly different powers. I wouldn't expect Google to be held to the same standard that I hold PG&E. Nor would I hold PG&E to the same restrictions I'd place on Google.
You made that up, or you checked stats?
A quick Google says that Facebook is still growing at about 5% and that Meta revenue is up a lot.
AFAIK there is no consensus that Facebook usage is declining - however that narritive gets told by people. Of course, it might depend on which country you look at.