Going on and on about the privacy implications is a distraction from the more real and more fundamental problem of corporate censorship. Nobody is harmed by ad targeting. We are all harmed by censorship.
Gamification is the nicotine, ad targeting is the tar of social media.
It’s also been documented that lack of privacy has a chilling effect in public discourse, whistleblowing, and other activities where you’d expect a reasonable level of privacy.
It’s not 100% privacy vs. none, but at the moment Facebook and Google are both very, very close to ultimate knowledge about an individual.