It all started because they needed to fill it up after the content shared by your friends is finished.

Well yeah, scrolling through and liking a picture of your friend's vacation and commenting "Adorable!" on a video of your cousin's toddler only gives you, say, 10 minutes to see ads, whereas getting fed an endless stream of progressively more intense and precisely-tuned content to tickle at your inner psyche (be it most susceptible to anger, lust, envy, greed etc) means you might spend hours on there scrolling past ads.

Well, in theory they could have just stuck to being a humble social media site, even if the traffic were to plateau or drop slightly. Something like what Craigslist did, but slightly more modern.

But of course if they'd done that Meta wouldn't be worth a hundred gazillion dollars now.