This is mostly about OP, not Facebook. The reason he sees tons of AI images of AI girls is because that's the kind of content he consumes on various Meta platforms. When I login to Facebook, I see none of that. So...
I am in a couple dozen active groups across a variety of topics - guitar, tech, TV shows, history, tabletop gaming, etc. - and 99% of posts are on-topic chatter by humans.
I prefer Reddit because it's longer-form content but with communities, it's about where there's a center of gravity - a subreddit, a FB group, a Discord, a traditional forum, etc. I go where the people are. And a lot of those people are on FB for some niches.
The "FB is nothing but AI slop and ads" is a myth. I have interesting conversations with people I don't personally know (in a real life sense) on FB every day.