A bunch of related thoughts:
1. How do we know FB will assign the scores fairly and objectively? We don't, of course, but is there any legal requirement or protection for users?
2. How is it like the Chinese government's social credit score? Right now Facebook says it's used for filtering user reports, but if FB becomes confident in it, what else might it be used for? Will they sell the score to advertisers and vendors so that they can filter customers? To credit score companies? To insurance companies and potential employers? Will it be used to 'nudge' mass behavior in certain directions? Is there anything preventing FB from doing those things?
3. As usual it's the powerful (FB, potentially their advertisers and other partners) who have information about ordinary people (FB users). Isn't it more important that ordinary people have trust information on the powerful? What is Mark Zuckerberg's trust score? Google's? Elon Musk's? Various politicians?
4. According to the article, FB isn't releasing the algorithm because it would enable users to game it (in fairness, I don't remember if FB said that or someone else). But those with resources and motivation, such as the high-end propagandists, will learn how to game it anyway, which can have at least two consequences: 1) Propaganda may become harder to identify, and 2) power will be even more concentrated in the hands of the few - FB, their partners, and those who can game the system.
5. As seems the norm for tech, the users have very little agency (other than to try to please the system); they are objects of technology and of those who control it. It bears repeating that we've come a long way from the world of free-as-in-speech systems and the idea that the goal is end-user control. In another context, an article posted to HN paraphrased Alan Kay as asking, 'Should the computer program the kid or should the kid program the computer?' [0] I think our society has answered that question for the present.
[0] http://wheels.org/spacewar/stone/rolling_stone.html