My guess is every metric is just getting diluted by bot activity but there's enough real users buying crap to give their advertising positive returns.

Engagement is great if you target a specific group. Don't need human content. It's ridiculously easy to start a Facebook page in a niche targeting a specific demographic, connect a site to it, unleash AI generated content, post it on FB and run ads. With enough traction, Facebook will pay you for making more content, while you extract money from your page followers. You're separating easy-to-influence boomers and conspiracy theorists from their money. It's disgusting, but it is ridiculously easy to make heaps of money with whatever content on Facebook.