Studies by mobile growth platform Branch revealed 25% of iOS users have opted in to Apple's App Tracking Transparency framework, Bloomberg reports. The lack of opt-ins is concerning for marketers of all sizes, from Facebook down to small- and medium-sized businesses, that rely on user tracking for advertising campaigns. "Smaller businesses are a casualty," Run DMG Media Buyer Gil David said. "I’m not really sure Apple fully thought that through, or they were aware of that and just thought, 'We don’t care. This is what we’re doing.'"