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They sell attention (usually via ads) by keeping users engaged and showing targeted advertising alongside content.
Engagement (watch time, clicks, reactions) is an easy-to-measure proxy for attention, which correlates with ad opportunities and revenue.
A feed is an ordered stream of content items (posts, videos, stories) selected and ranked for a user.
Following is mostly from accounts you follow; Recommended mixes in content predicted to interest you, often from non-followed accounts.
The social graph is who you follow/know; the interest graph is what you watch, click, and linger on—used to predict future interests.