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.
I could definitely believe that I used to click on more pictures of girls than boys back in high school and college when I actually used Facebook. But they would have been real pictures of people I was friends with.
To your point, I'm sure if I used the product more, the algorithm would get "better" according to what I engaged with.
Same issue in other social media btw, it’s probably too obvious in FB since it’s an old site with old audience, but if you go to instagram and the likes it’s all about thirst traps, which is a result of having a hypersexual society plus monetization.
The surprising effectiveness of Meta Ads for certain audiences as counter-intuitive as it seems is one example.