It gave those advertisers a great deal, charging a pittance to target ads based on the dossiers of nonconsensually harvested personal data they'd stolen from you.
Sellers became dependent on Facebook, too, unable to carry on business without access to those targeted pitches. That was Facebook's cue to jack up ad prices, stop worrying so much about ad fraud, and to collude with Google to rig the ad market through an illegal program called #JediBlue:
https://en.wikipedia.org/wiki/Jedi_Blue
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