Apple isn't holding Facebook hostage, it's simply allow FB users to relinquish giving Facebook their usage patterns for free.
For example, Facebook offers you to pay to "increase the reach" of your posts. This happens for example when you try to post a message to a Facebook group. If you want to make sure all members see the message, you have to pay.
This is a way that Facebook monetizes without tracking (targeting is based on people who opted in to join a group) and the users pay directly in the app (allowing Apple to get their 30% for the in-app payment).
With Apples changes, Facebook (and others) might focus more on strategies like that.
That actually sounds incredibly cynical, and criminal
Edit: Would the person who downvoted me within literally a minute of posting this please explain why?
If you wanted to send a letter to all of your friends in the traditional post, you’d have to pay 58 cents per person to do so, the business model hardly seems unusual.
Facebook tries to show you the most "interesting" posts. If you are a member of many groups, there are probably too many posts in your groups to show all of them in your newsfeed. So Facebook just picks whatever they think is interesting.
Or the poster can pay to have everyone see the post.
Anyway, I prefer this monetisation strategy to creepy tracking.