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.