Addiction is exactly what laws should be fighting, be it social media, drugs, or something else. In a free market, people should be free to choose where they spend their time and money, but people aren't free if they're addicted. Addiction causes them to spend their time and money on things which they wouldn't consent to, if it weren't for addiction. For Instagram, the harm is the lost time, and whatever else they could have done with that time. Addictive stuff disrupts the whole market / society, not just the individual.